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Article

The Influence of Roughness of Surfaces on Wear Mechanisms in Metal–Rock Interactions

by
Vlad Alexandru Florea
1,*,
Mihaela Toderaș
2 and
Ciprian Danciu
2
1
Department of Mechanical, Industrial and Transportation Engineering, University of Petrosani, 332006 Petrosani, Romania
2
Mining Engineering, Surveying and Civil Engineering Department, Faculty of Mines, University of Petrosani, 332006 Petrosani, Romania
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(2), 150; https://doi.org/10.3390/coatings15020150
Submission received: 12 January 2025 / Revised: 22 January 2025 / Accepted: 27 January 2025 / Published: 30 January 2025
(This article belongs to the Special Issue Friction and Wear Behaviors in Mechanical Engineering)

Abstract

:
The processes of rock excavation and processing involve intense mechanical stresses on cutting, displacing, and transporting tools, inevitably leading to the phenomenon of dry friction wear. The factors influencing the intensity and mechanisms of wear are complex and interdependent, being conditioned by the physical–mechanical properties of the rocks, the geometric characteristics and materials of the tools, as well as the cutting process parameters (cutting force, feed rate). Previous studies have mainly addressed the global aspect of wear without delving into the microstructural evolution of the contact surfaces during the friction process. In this paper, through controlled tribometric tests, we have investigated in detail the abrasive wear mechanisms of metallic materials in contact with different types of rocks, with an emphasis on the role played by surface roughness and the mineralogical properties of the rocks. Experimentally, we varied the applied forces and the number of friction cycles to simulate different working conditions and evaluate how these parameters influence wear intensity and surface morphology evolution. Microstructural analysis of the samples, combined with roughness measurements, allowed the identification of the predominant degradation mechanisms (abrasion, adhesion, fatigue) and their correlation with the material properties and the friction process parameters. The results have shown a strong correlation between the wear capacity of rocks and their petrographic properties, such as hardness, porosity, and hard mineral content. It was also found that the roughness of the contact surfaces plays an essential role in wear mechanisms, influencing both the initiation and propagation of its effects. Depending on the experimental data, we have developed a classification of rocks based on their abrasive potential and proposed criteria for the optimal adoption of materials and working parameters for the tools of technological equipment depending on the type of rock encountered. The results of this study can contribute to improving the durability of tools, as well as mining equipment, and reducing operating costs.

1. Introduction

The operation and maintenance of technological equipment in various fields, such as road and railway construction (including tunnels) and underground or surface mining, are influenced by their correct use according to specific working conditions. The interaction between machinery and rocks is a complex phenomenon, fundamental to a wide range of industrial activities, from mining and construction to materials processing. In this context, the wear of metallic elements in direct contact with rocks is a major problem with significant implications for the efficiency, operating costs, and lifespan of equipment.
Wear through friction, the phenomenon by which material is detached from the surface of a solid body in relative motion with another solid body, is the dominant mechanism in the case of machine-rock interactions. The main wear mechanisms [1] involved in this process are: abrasive wear, which occurs through the scratching and cutting of the surface due to hard particles in the rock acting as abrasives; adhesive wear, which occurs through the welding and breaking of micro-asperities on the contact surfaces, generating material transfer; fatigue wear, which occurs as a result of repetitive cyclic stresses, leading to the initiation and propagation of surface cracks; and corrosive wear, which is intensified by the presence of a corrosive environment (water, acids or other chemicals present in the rock). Wear mechanisms at metal–rock contact surfaces depend on the predominance of one of two categories of factors during the dislocation, transportation, and various processing and utilization methods of rocks. These factors are the properties and structure of metallic materials and rocks, respectively, and the parameters of the dislocation regime, transportation, processing, etc., of rocks.
The wear of metallic elements in contact with rocks is a complex process [2,3] influenced by various factors, including rock properties and material interactions, which significantly impact equipment performance [4,5] and operational costs [6,7,8]. Metallic wear in rock contact is influenced by rock hardness, abrasiveness, and metal properties [9,10]. Abrasion, adhesion, and fatigue are primary wear mechanisms [11]. Rock porosity can exacerbate wear by retaining abrasive particles and affecting lubrication [12,13,14,15]. Operating parameters, such as force, speed, contact duration, and environmental conditions, significantly impact wear mechanisms, influencing the rates of abrasion, adhesion, and fatigue [12,16,17]. For example, water can accelerate rusting, while lubricants can reduce friction and protect surfaces. To describe and predict the wear phenomenon, numerous mathematical models have been developed [18,19,20]. These models consider a variety of parameters, such as material hardness, applied force, sliding speed, and contact time.
The metal coating industry seeks advancements to enhance performance [5,21]. An example of coating effectiveness is the deposition of WC-12Co and WC-10Co4Cr by high-velocity oxy-fuel on additive manufactured maraging steel, reducing its abrasion wear, coefficient of friction, and corrosion [22]. Tribological studies on hoist cables reveal friction variations with strand crossing angles and directions [1,23,24]. High-temperature zones and diverse wear morphologies are observed under different conditions. Adhesive, abrasive, and fatigue wear mechanisms predominate. Scientific research should focus on the synergistic effects of rock properties, material interactions, and operating conditions on equipment efficiency.
The purpose of this research is to investigate the abrasive wear of metallic materials against different rock types using tribological tests. By correlating the wear behavior with the physical–mechanical properties of rocks (hardness, porosity, and mineralogical composition), we aim to develop a classification system for rocks based on their abrasiveness. The findings of this study will contribute to the selection of suitable materials and operating conditions for equipment operating in various geological environments, leading to improved component durability and reduced maintenance expenses.

2. Rock Classification and Its Relevance to Tool Selection

In the processes of mechanical rock excavation [25,26,27,28], through rotary and/or percussive drilling cutting, crushing, etc., wear phenomena of the tools equipped with the technological equipment are influenced by the abrasiveness, hardness, compressive strength, impact strength, grain size, moisture content, and coefficient of friction (depending on the state of the contact surface) of the rocks. The mechanical work performed by the tool increases with the compressive and impact strength of the rock, Table 1 [29,30,31,32].
The information presented in Table 1, Table 2, Table 3 and Table 4 provides a deeper understanding of the relationship between the mechanical properties of rocks and their impact on wear processes in metal–rock interactions. Rocks are extremely diverse materials with vastly different physical and chemical properties. To understand how they interact with metals, it is essential to classify and characterize them based on these properties. Rock classification provides a framework for establishing a direct link between the physical characteristics of rocks and their behavior during interactions with metals. Depending on the type of rock to be processed, suitable tool materials with adequate hardness and wear resistance can be selected. Knowledge of rock abrasiveness allows for the design of mining equipment with a longer service life. Table 1 provides a solid foundation for understanding how the intrinsic properties of rocks influence wear processes when they come into contact with metals. Rocks with higher compressive strength and impact resistance tend to generate sharper and harder particles during the wear process, thus intensifying the abrasive effect. A higher value indicates a harder rock, meaning that the generated particles will be more difficult to deform and will cause faster wear of the metal. The type of rock (igneous, sedimentary, metamorphic) influences both the hardness and mineral composition of the generated particles, thus affecting wear mechanisms. For example, granite, being harder and having a higher compressive strength, will generate sharper and harder particles, which will cause faster wear of the metal through abrasion. On the other hand, limestone, which has a much lower strength, will generate less sharp particles and cause slower wear but may promote other wear mechanisms, such as adhesion. The higher the roughness of the rock and the closer the mechanical properties of the rock are to those of a hard rock, the more intense the metal wear will be. The dominant type of wear mechanism depends on both the properties of the rock and the operating conditions (speed, pressure, lubrication). To minimize wear, it is essential to select metal materials with adequate hardness and abrasion resistance and to use lubricants and coolants to reduce friction and temperature at the metal–rock interface. The mechanical properties of rocks provide an important basis for understanding and predicting the behavior of materials under wear conditions. By correlating this information with experimental studies, more accurate models can be developed to evaluate the service life of metal components that come into contact with different types of rocks.
Theoretical and experimental research conducted in laboratory and field settings on rock-cutting processes has aimed at determining the stresses, shapes, and materials of cutting tools [32] but has focused less on tool wear as a function of rock properties.
In the literature [33], rocks have been classified into five classes based on their ’strength’, i.e., the hardness of the constituent elements (Mohs hardness) and compressive strength (Table 2). Mohs hardness increases with the compressive strength of the rock, as shown in Table 2, thus leading to higher tool wear in harder rocks. At the same time, there is M.M. Protodiakonov’s classification (Table 3), used primarily in drilling, which divides rocks into 10 strength classes (f); according to Protodiakonov’s classification, hard rocks have a high “f” coefficient, so drilling basalts, quartzites, or granites causes intense tool wear and high specific consumption. The abrasiveness (Table 4) of rocks Ä according to Oparin [34] and B according to Baron and Kunetsov [34] has been another criterion for their classification. The high abrasiveness of some rocks (Table 4) also leads to intense wear of the tools used to excavate them.
Table 4. Classification of rocks in terms of the rock abrasivity [34].
Table 4. Classification of rocks in terms of the rock abrasivity [34].
Abrasivity Group, JAbrasivity Index, AAbrasivity Index, BRock Characteristic
13−4.2<5Very low abrasiveness
24.2−65−10Very low abrasiveness
36−8.510−18Lower-than-average abrasiveness
48.5−1218−30Average abrasiveness
512−1730−45Higher-than-average abrasiveness
617−2445−65Abrasiveness
724−3465−90High abrasiveness
834−48>90Extreme abrasiveness
Generally, there is a positive correlation between Mohs hardness and compressive strength (Table 2). In other words, harder rocks (higher Mohs hardness values) typically have higher compressive strength. Soft rocks, with a Mohs hardness of 1–2 and a compressive strength below 10 MPa, are very easy to scratch and displace. They generate fine and soft particles during the wear process, which can lead to reduced abrasive wear of the metal. However, they may promote other wear mechanisms, such as adhesion. Semi-hard and hard rocks, with a Mohs hardness between 3 and 6 and a compressive strength between 70 and 250 MPa, are more resistant to scratching and displacement. Very hard rocks, with a Mohs hardness of 6 and a compressive strength above 250 MPa, are extremely resistant to scratching and displacement. They generate extremely hard and sharp particles, which can cause extremely rapid wear of the metal. The classification of rocks based on the degree of abrasiveness is related to the mechanical properties of the rocks (Table 3 and Table 4). Rocks with relatively low Mohs hardness and compressive strength also generate soft and slightly sharp particles, leading to low abrasiveness. Rocks with high Mohs hardness and compressive strength generate hard and sharp particles, leading to high abrasiveness. All of this has implications for metal–rock wear, including the following: (a) the selection of appropriate materials (if the metal comes into contact with hard rocks, it is necessary to select materials with high hardness and abrasion resistance); (b) the degree of lubrication (the use of lubricants can significantly reduce wear by reducing friction and temperature at the metal–rock interface); (c) cooling can help extend tool life by reducing temperature and the risk of adiabatic wear; (d) tool geometry (optimal design can reduce cutting forces and improve particle evacuation, thus reducing wear); (e) control of operating conditions can significantly influence the intensity of wear processes.
Rock excavation through mechanical cutting (using mining combines and ploughs, excavators) occurs through the chipping action of blades mounted on subassemblies designed for specific mining equipment used in underground mines or quarries. Knowledge of the rock’s cutting resistance is crucial for designing subassemblies and cutting tools. Mechanical cutting involves the interaction between the blade (cutting tool) and the rock, characterized by a system of forces (Figure 1).
The angles α, β, γ, and δ characterizing the cutting edge of the tool have values specific to each rock type and influence the tool wear pattern.
The schematic representation of the metal–rock interaction in Figure 1 allows us to better understand the mechanisms underlying the wear of cutting tools. The metal comes into direct contact with the rock, often a hard and abrasive material, and this contact generates considerable forces leading to plastic deformation and fragmentation of the material, i.e., the rock. The interaction between the cutting tool and the rock leads to progressive wear of the tool. The hard particles in the rock act as abrasives, scratching, and removing material from the tool surface. This wear process can lead to a decrease in tool performance and, eventually, the need for replacement. By correlating this representation with the information in Table 1, Table 2, Table 3 and Table 4 regarding rock properties, we can identify the factors that influence wear and make informed decisions to optimize processing and improve production efficiency.
The non-homogeneous wear patterns exhibited by the components of the machine tool lead to differential rates of deterioration. Consequently, to maintain operational efficiency and minimize repair costs, a condition-based maintenance strategy incorporating selective replacement and reconditioning of worn components is recommended. Understanding wear mechanisms is critical for optimizing the performance and reliability of mechanical components. F.T. Barwell’s [3] classification, which identifies four fundamental wear mechanisms—adhesion, abrasion, fatigue, and corrosion—provides a robust conceptual framework for analyzing wear phenomena. While these mechanisms can act independently, in most industrial applications, wear results from a complex interplay of these mechanisms. Consequently, a detailed evaluation of operating conditions and materials is necessary to identify dominant wear mechanisms and implement appropriate measures to prevent and mitigate wear [19]. Abrasive wear is caused by the presence of hard particles between contacting surfaces or by harder asperities on one of the contacting surfaces. This type of wear is easily recognizable by the presence of dispersed or oriented micro-cutting marks. Additionally, it can accelerate corrosive wear, as observed in mining tools such as drill rods, drill bits, and combine cutter blades and discs that operate under the action of cooling fluid jets.
The abrasive wear observed in the gear teeth was primarily caused by hard particles generated from the previous shearing of adhesive junctions and the detachment of hardened surface layers due to fatigue. These wear particles acted as cutting tools, removing material from the contacting surfaces and accelerating the wear process. The simplicity of the abrasive wear theory can be deceiving. In practice, the complex interplay of factors such as the size, shape, and hardness of abrasive particles, along with the sliding velocity and contact pressure, makes it difficult to accurately predict wear rates.
Bergbau-Forschung GmbH has established the influence of wear, specifically on the cost of cutting tools used in mining combines, as a function of rock properties [32]. When the tool wear factor exceeds 0.3, costs can increase rapidly, sometimes by a factor of 10, due to increased tool consumption.
In some cases, experimental methods used to evaluate rock abrasiveness, such as the Los Angeles Rattler Test [35], reproduce the rock–metal friction process differently compared to other procedures (e.g., Baron’s method). This method is primarily used because rock samples are subjected to impact loading, and it allows for the measurement of weight loss after 500 rotations in a rotating cylinder containing metal spheres. However, it requires numerous tests, especially when the rock mass exhibits significant variations in structure and composition.

3. Materials and Methods

3.1. Materials

The analyzed rocks belong to the category of plutonic and volcanic igneous rocks (acidic, intermediate, and basic) [36]. Microscopic analyses conducted on these rock types revealed a holocrystalline, hypidiomorphic-granular texture, ranging from slightly to sometimes porphyritic-vitreous, ophitic, a massive (non-oriented), compact texture, in some cases with fractures filled with carbonates and pyrite, and a mineralogical composition primarily consisting of quartz, orthoclase and/or basic plagioclase feldspars, augite, olivine, oxide minerals, pyrite, and secondary minerals (clay minerals, sericite, calcite), as shown in Table 5.

3.2. Estimation of Rock Hardness and Abrasivity

The calculated values for the considered rock types are presented in Table 6. Hardness was determined with a sufficient approximation based on the mineralogical composition. A weighted average of the hardness of the constituent minerals was calculated. The percentage content of each mineralogical component was multiplied by its hardness (determined using the Mohs hardness scale), and the sum of these products was divided by 100:
Δ = i = 1 n Δ i A i 100 ,
where Δ is rock hardness in Mohs scale, Δi is hardness of the constituent minerals in Mohs scale, and Ai is the percentage content of each mineral component in the rock mass.
The quantitative evaluation of abrasivity was highlighted by the abrasivity coefficient:
K a b r = i = 1 n g i 2 n ,   mg
where Kabr is rock abrasivity, mg; gi is weight loss of the standard bar in each pair of experiments (mg), and n is the number of paired tests.
For each rock type, the minimum, maximum, and average abrasivity were determined, with the average values of this parameter being presented in Table 6.
The rocks analyzed belong to the category of plutonic and volcanic igneous rocks (acidic, intermediate, and basic) [36]. Microscopic analyses conducted on these rock types revealed a holocrystalline, hypidiomorphic-granular texture, ranging from slightly to sometimes porphyritic-vitreous, a massive (non-oriented), compact texture, and a mineralogical composition consisting of quartz, orthoclase and/or basic plagioclase feldspars, augite, olivine, oxide minerals, pyrite, and secondary minerals (clay minerals, sericite, calcite).
From the analysis of the results, it is confirmed that there is a general trend for rocks with a higher abrasivity coefficient to also have a higher hardness. This is logical because harder minerals tend to scratch other materials more easily and, at the same time, can generate rougher surfaces. However, there are also exceptions to this rule. For example, the Soimos diorite has a similar hardness to the Soimos 1 granite but a lower abrasivity coefficient. This is explained by differences in mineralogical composition and rock structure [36].
Rocks with a high abrasivity coefficient may have a higher roughness due to the presence of hard minerals that form protrusions on the surface. Additionally, they may exhibit a network of fractures and cracks that increase roughness.
Rocks with a low abrasivity coefficient may present a smoother surface, with fewer irregularities, and may also have a more homogeneous and compact structure.
Rocks with a higher abrasivity coefficient tend to exhibit a greater increase in roughness after loading cycles. This suggests that more abrasive rocks have a greater tendency to wear and develop rougher surfaces during friction. An increase in load and number of cycles generally leads to an increase in roughness. This is evident because greater forces and a higher number of loading cycles will accelerate the wear process. The rock type influences both abrasivity and wear behavior. For example, granites and gabbro, in general, exhibit a greater increase in roughness compared to limestones and marbles.

3.3. Tribological Testing Analysis

Friction between two surfaces moving relative to each other causes wear, resulting in changes in surface roughness and progressive material loss [37,38,39,40,41]. Studies conducted by Oparin et al. [34] on rock abrasiveness (Ä) have shown that it is dependent on other structural and physical–mechanical properties. This relationship can be expressed as Ä = φ1(Ç), where Ç represents the rock type. The wear of a working tool (R) in technological equipment depends on the material properties (Ð) from which it is made, the rock properties (L), and the operating parameters (modes) (G) of the machine, namely R = φ2(Ð, L, G). Efficient interaction between the working tool and the rock is possible if the condition Ä ≪ R is met. The efficiency of the rock displacement process is influenced by accompanying phenomena, including thermodynamic effects due to dry friction at the contact surface between the working tool of technological equipment and the rock. The mechanical and structural characteristics of a working tool can be more easily designed and adapted to rock properties if a single parameter, such as abrasiveness Ä = φ1(Ç), which characterizes the rock, is used. The basic properties that characterize rock abrasiveness are particle size, hardness of mineral constituents, compressive strength, porosity, bond strength between rock particles, and moisture content.
The abrasive nature of a rock is directly correlated with the size and hardness of its constituent particles. This relationship can be mathematically expressed using the coefficient ψZ, as shown by the following equations:
ψ z = f D , k ,   s 1 ,
s 1 = 0.022 × e x p 0.5465 × R M a ,
where D characterizes the size of the rock particle, k is the coefficient accounting for the effect of particle hardness on rock abrasiveness, and RMa is the average Mohs hardness value of the mineral constituents.
The bonding material between particles in sedimentary rocks significantly influences surface roughness and hardness. Similarly, the unrecrystallized material filling the pores of igneous rocks directly impacts the roughness and hardness of surface pores, a relationship that can be quantified using the coefficient ψP:
ψ P = f ( P ,   s 2 ) ,
where P is the porosity of a rock, and s2 is the coefficient accounting for the effect of the rock’s Mohs hardness on its abrasiveness.
Rocks with a strong binding material allow for a more complete utilization of the potential of each particle at the contact surface with the displacing tool until the moment of particle detachment. Consequently, it can be considered that the greater the strength of the binding material, the higher the abrasiveness of the rock particles, which is expressed by the coefficient ψS:
ψ S = f ( K W ) ,
where KW is the coefficient that considers the variation in rock compressive strength with changing moisture content and porosity.
The set of Equations (1)–(4) collectively describes the abrasive capacity of the rock:
Ä = ψ z + ψ P + ψ S ,
where Ä is the dimensionless parameter expressing the qualitative measure of rock abrasiveness based on its physical and mechanical properties.
By comparing the abrasivities Ä (Oparin method) of rocks, determined from their physical and mechanical properties, with those obtained using the Baron method (B), for the same rocks, similarities can be observed (Table 4) [34].
Building upon the pioneering research of Oparin, V.N. and Tanaino, A.S. [34], the authors of this work aimed to deepen the understanding of tribological interactions between rocks and metals. To obtain quantitative and reproducible data, experiments were conducted on a TRB3 tribometer (manufactured by Anton Parr, Montreal, QC, Canada) (Figure 2). This instrument allowed for the simulation of the wear process under controlled conditions, enabling the evaluation of how different types of rocks, characterized by specific abrasivity indices, influence the degradation of a metal ball. Through this experimental approach, the goal was to obtain detailed information on wear mechanisms and to establish correlations between material properties and the tribological behavior of the system. The obtained results have significant implications for optimizing the performance of industrial equipment operating in abrasive environments, such as those encountered in the mining or construction industries.
Before conducting dry friction tribological tests, a detailed characterization of the rock surface roughness was conducted using a Sutronic S128 optical profilometer supplied by Taylor Hobson (Taylor Hobson Overseas Ltd., Leicester, UK). Through this instrument, two-dimensional surface profiles were obtained (Figure 3), which allowed for the evaluation of important parameters such as the average arithmetic roughness (Ra), the root mean square roughness (Rq), and the total profile height (Rt) of the rock sample surfaces during the tribometric tests that formed the basis of the analyses presented in the paper. These parameters have a significant influence on the tribological behavior of materials, affecting both the coefficient of friction and the wear rate. The obtained data were used to correlate the geometric characteristics of the surface with the results of the tribological tests, allowing for a better understanding of the interaction mechanisms between rock and metal.
In order to assess the effect of rock abrasivity on tool wear, a set of samples with abrasivity values ranging between 21 and 61 mg (as determined by the Baron method) was selected (Table 7). This method, which measures the mass loss of a metal standard after friction with the rock sample, provides a quantitative measure of the material’s abrasiveness [15,42]. The selected range of values corresponds to a domain where, in practice, a significant influence of abrasivity on tool performance has been observed. Thus, rocks with higher abrasivity values generate higher wear rates, changes in the tool’s geometric profile, and, consequently, a reduction in its service life [3,13,32,43,44]. Wear mechanisms are complex, involving both mechanical processes such as scratching and fracture as well as physicochemical processes like adhesion and diffusion [12]. Abrasive particles in the rock act as microscopic cutting tools, removing material from the tool surface and causing progressive damage [45,46,47,48].
Sample preparation was conducted as follows: cores were extracted (in the Rock Mechanics Laboratory of the University of Petrosani) from rock blocks sampled from the mentioned locations and were subsequently cut using a diamond saw; the rock samples obtained from the cores had dimensions corresponding to the adjustable clamping system of the tribometer. The experimental setup consisted of a pin-on-disc tribometer in which a 100Cr6 alloy steel ball (bearing steel, mass to 0.8875 g, hardened to 63 HRC through oil quenching from 850–860 °C), with well-defined hardness and wear resistance properties, was used as a counterbody for the rock samples. This configuration allowed for the controlled simulation of the interaction between a cutting tool (the steel ball) and the workpiece (the rock sample).
The TB3 tribometer permits the adjustment of testing regime parameters (Figure 4): the magnitude of the static load exerted on the rock sample by the steel ball, the rotational speed, and the number of rotational cycles of the rock. The tribological tests were conducted under varying normal loads of 1 N and 5 N, simulating different rock processing conditions. The sliding speed was maintained at a constant 400 rpm, while the test duration was set at 500 and 1000 cycles to evaluate the time evolution of wear phenomena.
Post-test analyses involved the following: (a) quantifying the wear volume of the steel ball using optical or profilometric measurements of the wear area (Figure 5); (b) characterizing the surface morphology and roughness of the rock sample (Figure 6) using a profilometer to assess friction-induced modifications and identify the dominant wear mechanisms (abrasion, adhesion, fatigue); and (c) measuring the coefficient of friction during the test to correlate friction force with wear severity and to identify stable and unstable friction regimes (Figure 7).
The tribological behavior of various Romanian rock types, including limestone, andesite, marble, light andesite, granite, sandstone, and gabbro, sourced from different locations such as Cazanesti, Şoimos, Săvârşin, Roşia Montană, Certej-Valea Căpitanului, Deva-Dealul Motor, Brad-Criscior, Săcărâmb, Albini-Haneş, Roşia Poieni-Dealul Jgheabului, Brănişca and Dobra, was evaluated through controlled friction tests using a pin-on-disc tribometer (Figure 8). Tribological parameters such as surface roughness, coefficient of friction, and material removal volume (ball wear) were influenced by both the intrinsic properties of the rocks (hardness, porosity, mineralogical composition) and the testing conditions (normal load, sliding speed, number of cycles).
The influence of loading conditions on the wear behavior of various materials was investigated in this study. By varying the applied normal load and the number of loading cycles, different wear scenarios encountered in industrial applications were simulated. Morphological analysis of the contact surfaces, performed using profilometry, allowed the identification of the predominant wear mechanisms (abrasion, adhesion, fatigue) and their correlation with the material properties and testing conditions.

4. Results

The tribological test results, including changes in surface roughness and coefficient of friction for various rock types, are summarized in Table 8 for 500 and 1000 rotation cycles under normal loads of 1 N and 5 N, respectively. The corresponding wear volumes of the metallic ball during dry friction are shown in Table 9 for the same rocks and testing conditions. The results of the tribological tests conducted on the various rock samples revealed significant differences in their wear behavior. These showed that the wear rate increased with increasing normal load and number of cycles and that the dominant wear mechanism varied depending on the material properties and testing conditions.
The data presented in Table 8 and Table 9 were used to generate the graphs (Figure 9, Figure 10 and Figure 11) illustrating the influence of load value and number of cycles on the wear of the metal ball and the roughness of rocks with varying abrasivities. Rocks with high abrasivity, such as andesite and granite, exhibited significant variations in roughness depending on the applied load and number of cycles; the variation in granite roughness was 54.35% for a load of 5 N and 1000 cycles compared to 17.24% for 1 N and 500 cycles, with a metal ball wear of 0.7887% and 0.3718%, respectively. In the case of limestone, the variation in its roughness was much lower, namely 8.51% for a load of 5 N and 1000 cycles compared to 4.35% for 1 N and 500 cycles, with an impact on metal ball wear of 0.3606% and 0.1239%, respectively.
The results obtained demonstrate a significant correlation between rock abrasiveness and parameters such as surface roughness, wear of metal components in mining equipment, and, consequently, optimal operating conditions. The results indicate that, for a constant rock abrasiveness, the variation in surface roughness and metal element wear changes significantly depending on the applied load and the number of operating cycles.
In this context, it should be emphasized that to ensure adequate productivity in the exploitation of highly abrasive rocks, it is necessary to size the technological equipment with a significantly higher installed power compared to those intended for rocks with low abrasiveness. This requirement is justified by the need to overcome the higher cutting and crushing resistance of hard rocks. In the case of highly abrasive rocks, the feed rate of the working elements of the same equipment must be reduced to diminish the intensity of abrasive and adhesive wear processes. The feed rate, which also determines the number of cycles of the working elements of the same equipment, must be lower for those operating in highly abrasive rocks than for those operating in rocks with low abrasiveness. Consequently, optimizing technological parameters based on rock abrasiveness is a crucial factor in enhancing the efficiency and durability of mining processes, as well as reducing production costs.
A comparative analysis of the two andesite types revealed significant differences in the wear of metal components. These variations can be primarily attributed to the combined influence of compressive strength, characteristics of the intergranular bonding matrix, and the hardness of the constituent minerals. The results obtained suggest a complex relationship between the petrographic properties of andesite and the wear behavior of contacting materials. The petrographic heterogeneity of the two andesite types analyzed led to significant differences in abrasive wear behavior.
The outlined aspects regarding the influence of rock abrasiveness on surface roughness modifications during dry friction, including metal ball wear, provide guidelines for designing the technical characteristics of technological equipment and their subassemblies based on operating conditions (rock types). The hard particles within the rocks act as abrasive elements, capable of removing material from the metal surface through scratching. During this process, the interactions between the contacting surfaces can generate a certain amount of heat and local stresses, which promote adhesive wear and fatigue processes [16,17].
It is noteworthy that rocks with low abrasiveness (limestone, sandstone) have a reduced impact on metal ball wear, but at the same time they significantly modify their roughness. Consequently, the steels required for the displacement tools of technological equipment should not contain alloying elements and should have low specific consumption costs. Rocks with high abrasiveness necessitate the use of alloyed steels for displacement tools that are subject to excessive wear.
Materials selected for tools in contact with highly abrasive rocks (granite, gabbro, andesite) should be alloyed steels containing manganese, chromium, or molybdenum, or they should have their active surfaces protected with elements made of hard alloys such as tungsten carbides. In addition to alloyed steels, engineering ceramics, and composite materials can offer advantageous solutions in certain applications due to their wear resistance and high-temperature performance [5,32,36]. Material selection should be based on a cost-benefit analysis, considering both the initial material cost and long-term operating costs.

5. Discussion

The interdisciplinary investigation of rock–metal interactions has revealed the crucial role of surface roughness in wear phenomena. The experimental data presented in Table 8 emphasize the necessity of a multidisciplinary approach, integrating geological, materials science, and tribological perspectives, to comprehensively understand the underlying physical mechanisms. These findings (Table 8) establish a strong correlation between alterations in rock surface roughness and wear processes during dry friction with a metal component. The tribological analysis of rock–metal interactions has demonstrated that the evolution of rock surface roughness serves as a robust indicator of the wear behavior of tribological systems [45].
Roughness exhibited a positive correlation with both the number of cycles and load magnitude, suggesting the formation of surface asperities due to wear. Rock type significantly influenced wear behavior, with granites and gabbros demonstrating higher roughness increases compared to limestones and marbles. This variation can be attributed to differences in mineralogical composition, structure, and hardness [46,47]. The coefficient of friction was found to be dependent on rock type, load, and the number of cycles. A positive correlation between roughness and coefficient of friction was observed.
The wear rate of a metal interacting with a rock is directly related to the rock’s mineralogical composition [45,46,47,48]. Granites, with their high quartz and feldspar content, are highly abrasive and can cause rapid wear of metal components. This understanding is crucial for selecting appropriate materials and designing equipment for applications involving rock–metal interactions. Our findings align with the results of [46], who reported that quartz sand grains caused the most significant wear in sandstones. Similarly, we observed that granites, with their high quartz content, exhibited the highest wear rates. The presence of orthoclase and plagioclase feldspars, biotite, opaque minerals, and secondary minerals in granites likely contributed to this increased wear. Limestone, primarily composed of calcite, is softer and less abrasive than granite. However, under certain conditions, limestone can contain impurities that increase its abrasiveness. Plagioclase feldspar, being a relatively hard mineral, contributes to the overall hardness of the rock but can generate sharp fragments during wear, intensifying the abrasive effect. Hornblende, an amphibole mineral commonly present in andesitic rocks as elongated crystals, can increase the rock’s roughness, especially if the crystals are oriented perpendicular to the surface. The vitreous microlitic paste, kaolinite, sericite, calcite, and pyrite, as secondary minerals and amorphous components, can influence the rock’s wear resistance by filling pores and fractures, thus altering the material’s mechanical behavior. Regarding the Rosia Poieni andesite, the presence of kaolinite and sericite, soft and friable minerals, may indicate more advanced alteration of the rock and slightly reduced wear resistance compared to the Albini-Haneș andesite. On the other hand, the presence of calcite could influence the rock’s chemical behavior in contact with metals, accelerating certain corrosion processes. The Albini-Haneș andesite contains microlites and volcanic glass, suggesting a finer and more homogeneous texture, which could lead to initially lower roughness. However, the presence of opaque and secondary minerals may influence the rock’s behavior over time. The rock type (granite, diorite, gabbro, andesite, basalt) influences both hardness and abrasivity coefficient. For instance, granites and diorites, in general, have higher values of these parameters compared to andesites and basalts. Even within the same rock type (for example, andesite), there is significant variability in the values of hardness and abrasivity coefficient. This is due to differences in mineralogical composition, structure, and geological history of each rock.
A classification of rocks based on their surface roughness evolution during dry friction with a metal element enables design and operational engineers to make informed decisions regarding the selection of metallic materials for technological equipment components (such as displacement tools) and their operating parameters. Concurrently, it allows for the evaluation of a metal element’s wear behavior (Table 10) when in contact with various rocks under dry friction conditions.
The classification presented in Table 10 highlights the behavior of rock surfaces subjected to dry friction with metals during dislocation, transportation, and various processing and utilization methods. As observed during tribological tests, which allow the evaluation of the state of contacting surfaces while considering both the properties of rocks and metals, as well as the test regime parameters, roughnesses change by more than 40%–50% in the case of granites (54.35%) or andesites (44.9%) denoted as having “increased roughness”. Based on the classification outlined in Table 10, rocks can be categorized into five distinct groups depending on the roughness of their surfaces when in contact with metallic elements:
-
Rocks with very low roughness, where roughness increase is below 10%;
-
Rocks with low roughness, where roughness increase is between 10%–25%;
-
Rocks with medium roughness, where roughness increase is between 25%–40%;
-
Rocks with high roughness, where roughness increase is between 40%–55%;
-
Rocks with very high roughness, where roughness increase is above 55%.
Our findings align with the results of [45], who reported that quartz sand grains caused the most significant wear in sandstones. Similarly, we observed that granites, with their high quartz content, exhibited the highest wear rates. The presence of orthoclase and plagioclase feldspars, biotite, opaque minerals, and secondary minerals in granites likely contributed to this increased wear. The wear rate of a metal interacting with a rock is directly related to the rock’s mineralogical composition [45,46,47,48].
Quartz, due to its high hardness and brittle nature, generates sharp edges during abrasion, leading to significant material removal. Similarly, [49] highlighted the abrasive properties of feldspar minerals, particularly plagioclase, which can exhibit cleavage planes that contribute to wear.
The platy morphology and inherent cleavage planes of biotite contribute to increased wear in metals because these features promote material removal [50] studied the impact of opaque minerals, such as magnetite and ilmenite, on the wear of materials and reported that these minerals can act as micro-abrasives, accelerating wear processes.
Limestone, primarily composed of calcite, is softer and less abrasive than granite. However, under certain conditions, limestone can contain impurities such as quartz, silica, and other impurities, which can significantly increase its abrasiveness. The presence of silica impurities in limestone can significantly enhance its abrasive properties, leading to increased wear rates in contacting materials.
Plagioclase feldspar, being a relatively hard mineral, contributes to the overall hardness of the rock but can generate sharp fragments during wear, intensifying the abrasive effect. This finding is supported by the work of [51], who demonstrated that the presence of sharp-edged mineral fragments within a rock matrix significantly increases its abrasive potential.
Hornblende, an amphibole mineral commonly present in andesitic rocks as elongated crystals, can increase the rock’s roughness, especially if the crystals are oriented perpendicular to the surface. Referring to the influence of hornblende on the wear behavior of metals, it was found that the presence of elongated hornblende crystals can significantly increase the surface roughness of contacting materials.
The vitreous microlitic paste, kaolinite, sericite, calcite, and pyrite, as secondary minerals and amorphous components, can influence the rock’s wear resistance by filling pores and fractures, thus altering the material’s mechanical behavior. Scholars [36,46,47,48] studied the influence of secondary minerals on the wear behavior of rocks and reported that the presence of clay minerals such as kaolinite and sericite can reduce the rock’s hardness and increase its wear rate [49,50,51]. They investigated the influence of weathering and alteration on the wear behavior of rocks and found that advanced alteration can significantly reduce the rock’s hardness and increase its wear rate. On the other hand, the presence of calcite could influence the rock’s chemical behavior in contact with metals, accelerating certain corrosion processes.
Regarding the Rosia Poieni andesite, the presence of kaolinite and sericite, soft and friable minerals, may indicate more advanced alteration of the rock and slightly reduced wear resistance compared to the Albini-Haneș andesite. The Albini-Haneș andesite contains microlites and volcanic glass, suggesting a finer and more homogeneous texture, which could lead to initially lower roughness. However, the presence of opaque and secondary minerals may influence the rock’s behavior over time. A comparison of Roșia Poieni andesite and other andesites, such as Albini-Haneș andesite, highlights substantial variations in mineralogical composition and textural features. The elevated content of soft minerals, including kaolinite and sericite, within Roșia Poieni andesite is suggestive of more extensive alteration and, consequently, a slightly diminished resistance to wear.
The novel research approach presented in this study, along with the obtained results, is grounded in the observation that the rock classification currently used in engineering applications primarily focuses on the hierarchical arrangement of rocks based on their physical and mechanical properties. The proposed classification in Table 8 offers the potential to reduce costs associated with the design, procurement, installation, operation, and maintenance of technological equipment used in mining advancement works, quarry excavation, drilling, and tunnel excavation.
Wear track profiles provide essential information about the geometry and morphology of contacting surfaces, enabling the identification of dominant wear mechanisms. Optical micrographs allow for a detailed analysis of the material microstructure and wear-induced modifications, such as the formation of microcracks or the deposition of foreign particles. However, a comprehensive analysis of the microstructural changes associated with wear was not conducted in this study and will be the subject of future investigations.

6. Conclusions

This study presents a detailed analysis of the interaction between rocks and metals under dry friction conditions, with a particular focus on the phenomenon of abrasive wear. The results can be utilized to enhance the performance and durability of equipment employed in various industries, including mining, construction, and materials processing. The following conclusions were drawn:
  • Regarding the correlation between rock properties and wear, it has been demonstrated that there exists a direct link between the physical and mechanical properties of rocks (hardness, porosity, mineralogical composition) and their ability to induce wear on metallic surfaces. Rocks with higher abrasivity lead to a faster wear rate of metal tools.
  • The surface roughness of the rock significantly influences wear mechanisms. A higher roughness can accelerate the wear process by facilitating adhesion between rock particles and the metallic surface.
  • There is significant variability in the results, even for the same rock type. This can be attributed to differences in structure, mineralogical composition, and the presence of internal defects. The increase in roughness is explained by mechanisms such as microchipping, abrasion, and the formation of surface cracks and fissures. Rock roughness is influenced by a multitude of factors beyond abrasivity. Factors such as crystalline structure, fractures, pores, and weathering and erosion processes play a significant role.
  • In this study, a novel classification of rocks is proposed, categorized by their capacity to induce wear. This classification is grounded in experimental measurements and an analysis of the physical and mechanical properties of rocks. The proposed classification can be instrumental in selecting optimal materials for the cutting tools of technological equipment and optimizing operational parameters.
  • Abrasive wear mechanisms can significantly degrade the service life and performance of equipment utilized in rock excavation operations. Suboptimal material selection and process parameters can exacerbate maintenance costs and curtail productivity.
  • A thorough understanding of wear phenomena necessitates an interdisciplinary perspective encompassing geological, materials science, and tribological principles.
  • The outcomes of this investigation can be leveraged to optimize rock-cutting and processing procedures by facilitating the selection of suitable tool materials and the establishment of optimal process parameters. Through the minimization of tool wear, significant reductions in operational and maintenance expenses can be attained.
Knowledge of the rock–metal friction coefficient is of paramount importance in the extractive industry, particularly in selecting appropriate cutting and excavation equipment and optimizing extraction processes. It is also crucial for the design and manufacturing of machinery and equipment that come into contact with rocks. The friction coefficient between rock and metal plays a significant role in determining the specific energy required for various operations in the extractive industry. This relationship is complex and influenced by numerous factors. Future studies will delve deeper into the influence of these factors and the friction coefficient on dry metal–rock friction processes.

Author Contributions

Literature review and analysis, C.D. and M.T.; methodology, V.A.F. and M.T.; writing M.T. and V.A.F.; experiments, C.D. and V.A.F.; results analysis, C.D., V.A.F., and M.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The significance of tool geometry in the rock excavation process and the forces involved: (a) interaction between the cutting tool and the rock surface: Fx—tangential force; Fy—normal force; vt—cutting speed; α—clearance angle; β—rake angle; γ—cutting angle; δ—edge angle; (b) cross-section of the cutting zone: Fz—lateral force relative to the tool path; h0—cutting depth; ψ—chip rupture angle; S0—cross-sectional area of the rock chip; 1—tool; 2—rock; 3—chip.
Figure 1. The significance of tool geometry in the rock excavation process and the forces involved: (a) interaction between the cutting tool and the rock surface: Fx—tangential force; Fy—normal force; vt—cutting speed; α—clearance angle; β—rake angle; γ—cutting angle; δ—edge angle; (b) cross-section of the cutting zone: Fz—lateral force relative to the tool path; h0—cutting depth; ψ—chip rupture angle; S0—cross-sectional area of the rock chip; 1—tool; 2—rock; 3—chip.
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Figure 2. Work stand: 1—TRB3 tribometer; 2—Sutronic S128 profilometer; 3—rock sample; 4—static partner (steel ball); 5—loading.
Figure 2. Work stand: 1—TRB3 tribometer; 2—Sutronic S128 profilometer; 3—rock sample; 4—static partner (steel ball); 5—loading.
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Figure 3. Roughness parameters of the rock sample surface measured using a Sutronic S128 profilometer.
Figure 3. Roughness parameters of the rock sample surface measured using a Sutronic S128 profilometer.
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Figure 4. Test regime parameters for the TRB3 tribometer.
Figure 4. Test regime parameters for the TRB3 tribometer.
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Figure 5. Wear assessment of the metal ball: (a) through mass loss measurement; (b) by measuring the wear track.
Figure 5. Wear assessment of the metal ball: (a) through mass loss measurement; (b) by measuring the wear track.
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Figure 6. The tested rock sample surface.
Figure 6. The tested rock sample surface.
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Figure 7. Variation of the coefficient of friction on the rock sample surface.
Figure 7. Variation of the coefficient of friction on the rock sample surface.
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Figure 8. Rock samples tested using a TRB3 tribometer: 1—limestone; 2—Roșia Poieni andesite; 3—marble; 4—granite; 5—Albini Haneș andesite; 6—gabbro; 7—sandstone.
Figure 8. Rock samples tested using a TRB3 tribometer: 1—limestone; 2—Roșia Poieni andesite; 3—marble; 4—granite; 5—Albini Haneș andesite; 6—gabbro; 7—sandstone.
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Figure 9. Evolution of rock surface roughness with rock abrasiveness.
Figure 9. Evolution of rock surface roughness with rock abrasiveness.
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Figure 10. The wear rate of a ball as a function of rock abrasiveness.
Figure 10. The wear rate of a ball as a function of rock abrasiveness.
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Figure 11. The variation of the friction coefficient with rock abrasiveness.
Figure 11. The variation of the friction coefficient with rock abrasiveness.
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Table 1. Classification of rocks and mineral resources according to drilling resistance (according to Lares).
Table 1. Classification of rocks and mineral resources according to drilling resistance (according to Lares).
Rock
Category
Nature of Rocks and Useful Mineral SubstancesMechanical Work Input Necessary for the Perforation of a 1 cm3 Cavity (daNm)Compressive Breaking Strength (MPa)Impact Strength (MPa)
IExtremely hard and compact quartzites1703003.0
IIExtremely hard basalts, andesites, diabases, diorites, and highly compact conglomeratic granulites1552402.4
IIIVery hard granites, quartzites, and siliceous schists1352202.2
IVVery hard grauwacke, gneisses, basalts, porphyries, and amphibolites1182002.0
VHard granites and gneisses, syenites, porphyrites, and amphibolites1031801.8
VIVery hard limestones and sandstones, and hard conglomerates931601.6
VIIHard granites, semi-hard granites, grauwacke, very hard siderite, and sandstones801401.4
VIIIHard limestones, semi-hard gneisses and porphyrites, and hard schists701201.2
IXMarble, semi-hard limestones, dolomites, hard siderite, and magnesite581001.0
XSemi-hard siderites, soft limestones, and semi-hard sandstones47800.8
XISandy shales, stratified soft sandstones35600.6
XIIHard argillaceous shales, soft sandstones, and gypsum22400.4
XIIICoal10200.2
Table 2. Classification of rocks according to the strength of their constituent elements and uniaxial compressive strength.
Table 2. Classification of rocks according to the strength of their constituent elements and uniaxial compressive strength.
Rock ClassMohs HardnessCompressive Strength (MPa)
Very soft rocks1–2<10
Soft rocks2–310–70
Semi-hard rocks3–570–150
Hard rocks5–6150–250
Very hard rocks6>250
Table 3. Classification of rocks and useful minerals according to their resistance to drilling and excavation (Protodiakonov).
Table 3. Classification of rocks and useful minerals according to their resistance to drilling and excavation (Protodiakonov).
Rock CategoryDegree of HardnessThe Nature of Rocks and Useful MineralsStrength
Coefficient, f
IExtremely hard rocksQuartzites and basalts, the hardest and most compact20
IIVery hard rocksVery hard granitic rocks, quartz porphyries, very hard granites, quartz schists, less hard quartzites, very hard sandstones and limestones15
IIIHard rocksCompact granite and granitic rocks, very hard limestones and sandstones, very hard conglomerates, very hard iron ores10
Hard limestones, softer granite, hard sandstones, hard marbles, dolomites, pyrites8
IVFairly hard rocksNormal sandstones, iron ore6
Sandy shales, shaley sandstones5
VSemi-hard rocksHard clay shale, softer sandstones, and limestones, weakly consolidated conglomerates4
Different, slightly hard shales, compact marl3
VIModerately soft rocksSoft shale, very soft limestone, chalk, rock salt, gypsum, frozen ground, anthracite, ordinary marl, weathered sandstone, stony ground2
Less soft stony ground, weathered shale, compacted gravel and ballast, hard coal, hardened clay1.5
VIISoft rocksCompact clay, soft coal, hard alluvium, clayey soils1
Slightly sandy clay, loess, gravel (ballast), friable coal0.8
VIIIEarthy rocksVegetal soil, Peat, clayey soil, wet sand0.6
IXWeathered rocksSand, debris, fine gravel, fill soil, lump or crushed coal0.5
XFlow rocksQuicksand, swampy terrain, saturated loess, and other waterlogged and muddy lands0.3
Table 5. Mineralogical characterization of the studied rocks.
Table 5. Mineralogical characterization of the studied rocks.
Rock NameStructureTextureMineralogical CompositionSection
Soimos Granite 1Holocrystalline, granular hypidiomorphic, slightly porphyriticUnoriented (massive), compactQuartz, orthose, plagioclase feldspar, biotite, opaque minerals, sericite, kaoliniteCoatings 15 00150 i001
Soimos Granite 2Holocrystalline, hypidiomorphic granular, slightly porphyroidicUunoriented (massive), compactQuartz, orthose, plagioclase feldspar, biotite, opaque minerals, secondary mineralsCoatings 15 00150 i002
Săvârşin GraniteHolocrystalline, granular hypidiomorphic, slightly porphyroidicUnoriented (massive), compactQuartz, orthoclase feldspar (orthose), acid plagioclase feldspar, biotite, zircon, opaque minerals, sericite, kaolinite, limoniteCoatings 15 00150 i003
Soimos DioriteHolocrystalline, granular hypidiomorphicUnoriented (massive), compactIntermediate plagioclase feldspar, pyroxenes (augite and diopside) biotite, quartz, opaque minerals, limoniteCoatings 15 00150 i004
Cazanesti GabbrouHolocrystalline, hypidiomorphic and allotriomorphicUnoriented (massive), compactBasic plagioclase feldspar, augite, olivine, secondary minerals, saussuriteCoatings 15 00150 i005
Albinii-Haneș AndesiteHypocrystalline (semicrystalline), porphyry vitreousUnoriented (massive), compactNeutral plagioclase feldspar, hornblende, microlitic, volcanic glass, secondary and opaque mineralsCoatings 15 00150 i006
Roșia Poieni AndesiteHypocrystalline (semicrystalline), porphyry vitreousUnoriented (massive), compactNeutral plagioclase feldspar, hornblende, microlitic vitreous paste, kaolinite, sericite, calcite, pyriteCoatings 15 00150 i007
Table 6. Values of rock hardness and abrasivity (by Baron method).
Table 6. Values of rock hardness and abrasivity (by Baron method).
Rock TypeHardness, ΔAbrasivity Coefficient, mg
Soimos Granite 15.88856.43
Soimos Granite 26.52660.86
Săvârşin Granite6.49555.71
Soimos Diorite5.84047.93
Cazanesti Gabbrou5.35543.25
Albinii-Haneș Andesite5.02841.10
Roșia Poieni Andesite4.95132.56
Table 7. Mechanical behavior of rocks subjected to frictional wear.
Table 7. Mechanical behavior of rocks subjected to frictional wear.
Rock NameMohs
Hardness, Δ
Abrasivity (Baron Method), Kabr (mg)Protodiakonov Strength Coefficient, fCompressive Strength, σrc (N/mm2)
Gabbro5.35543.2515.017150.17
Granite6.52660.8621.222212.22
Albinii-Haneș Andesite5.02841.1014.240142.4
Roșia Poieni Andesite4.95132.5612.963129.63
Limestone3.52621.837.8178.11
Sandstone4.53630.1811.04110.43
Marble4.02530.6311.24112.42
Table 8. Changes in surface roughness values of rocks during dry friction.
Table 8. Changes in surface roughness values of rocks during dry friction.
Rock NameLoad
N
Number of
Cycles
Coefficient of
Friction
Initial Ra, μFinal Ra, μRa Increasing, %
Albini Haneș Andesite15000.31186.47.517.19
110000.49926.17.421.31
55000.48136.98.624.64
510000.57066.68.325.76
Limestone15000.32124.66.417.39
110000.57844.65.928.26
55000.52914.26.247.62
510000.57424.06.357.50
Gabbro15000.40465.35.89.43
110000.41555.46.418.52
55000.39224.04.820.00
510000.41454.35.527.91
Granite15000.39555.86.26.90
110000.39655.25.811.54
55000.49364.24.814.29
510000.50484.65.315.22
Sandstone15000.093817.622.628.41
110000.205219.225.733.85
55000.070016.222.840.74
510000.171614.122.056.03
Marble15000.01255.96.48.47
110000.07987.07.811.43
55000.12628.29.515.85
510000.15147.19.026.76
Roșia Poieni Andesite15000.59395.45.99.26
110000.56424.34.811.63
55000.53474.55.317.78
510000.53714.95.920.41
Table 9. Changes in metallic ball wear values.
Table 9. Changes in metallic ball wear values.
Rock NameLoad
N
Number of CyclesCoefficient of FrictionFinal Mass of the Ball (100Cr6), gReduction in Ball Mass (100Cr6), %
Albini Haneș Andesite15000.31180.88590.1803
110000.49920.88180.6423
55000.48130.88520.2592
510000.57060.88120.7099
Limestone15000.32120.88640.1239
110000.57840.88460.3268
55000.52910.88590.1803
510000.57420.88430.3606
Gabbro15000.40460.88510.2704
110000.41550.88230.5859
55000.39220.88460.3268
510000.41450.88140.6873
Granite15000.39550.88420.3718
110000.39650.88110.7211
55000.49360.88360.4394
510000.50480.88050.7887
Sandstone15000.09380.88670.0901
110000.20520.88470.3155
55000.07000.88620.1465
510000.17160.88380.4169
Marble15000.01250.88690.0676
110000.07980.88630.1352
55000.12620.88650.1127
510000.15140.88540.2366
Roșia Poieni Andesite15000.59390.88530.2479
110000.56420.88310.4958
55000.53470.88480.3042
510000.53710.88210.6085
Table 10. Classification of rocks based on surface roughness when in contact with metal elements.
Table 10. Classification of rocks based on surface roughness when in contact with metal elements.
Rock ClassificationRock NameRock Roughness* Roughness Increase %Abrasivity, mgBall Wear (Mass Loss) %Dislodging Tools
IGraniteVery rough rocks54.3560.860.7887Excavator/loader teeth, bucket teeth
IIRoșia Poieni Andesite Highly rough rocks44.932.560.6085Excavator/loader teeth, bucket teeth
IIIGabbroRocks with average surface roughness37.2143.250.6873Drill bits
Albini Haneș Andesite 36.3641.100.7099excavator/loader teeth, bucket teeth
Marble26.7630.630.5366drill bits
IVSandstoneRocks with low surface roughness20.5730.180.4169Excavator/loader teeth, bucket teeth
VLimestone Rocks with low surface roughness8.5121.830.3606Drill bits
* Note: The increase in roughness and wear of the ball (mass loss) was considered for a 5 N load and 1000 cycles.
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Florea, V.A.; Toderaș, M.; Danciu, C. The Influence of Roughness of Surfaces on Wear Mechanisms in Metal–Rock Interactions. Coatings 2025, 15, 150. https://doi.org/10.3390/coatings15020150

AMA Style

Florea VA, Toderaș M, Danciu C. The Influence of Roughness of Surfaces on Wear Mechanisms in Metal–Rock Interactions. Coatings. 2025; 15(2):150. https://doi.org/10.3390/coatings15020150

Chicago/Turabian Style

Florea, Vlad Alexandru, Mihaela Toderaș, and Ciprian Danciu. 2025. "The Influence of Roughness of Surfaces on Wear Mechanisms in Metal–Rock Interactions" Coatings 15, no. 2: 150. https://doi.org/10.3390/coatings15020150

APA Style

Florea, V. A., Toderaș, M., & Danciu, C. (2025). The Influence of Roughness of Surfaces on Wear Mechanisms in Metal–Rock Interactions. Coatings, 15(2), 150. https://doi.org/10.3390/coatings15020150

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