Towards Self-Lubricating Effect of In Situ Iron Tungstate in Rubbing WC/Mn13 Steel Cermet against a HSS Steel

Abstract

We study the mechanism of formation of the multilayer structure of the subsurface regions of WC/Mn13 steel cermets after sliding on a steel disk at speeds from 10 to 37 m/s and contact pressure of 5 MPa in order to elucidate the fundamental role of the processes of tribooxidation on a worn surface in the formation of the tribomechanical properties of a large family of similar W/Fe-containing materials. It was shown that the maximum antifriction effect of WC/Mn13 steel cermets under conditions of high-speed sliding was due to in-situ generated FeWO₄ that provided friction coefficient drop from ~0.17 to ~0.07 when sliding at 10 and 37 m/s, respectively. The sliding speed had its effect on the subsurface structure and tribolayer thickness, so micron-sized, mechanically mixed 3–4 μm-thick layers (MML) were generated in sliding at 10 and 20 m/s, whose wear occurred mainly by subsurface fracture and delamination of tile-shaped wear particles. Continuous tribolayers with 10–15 μm thickness were generated at 30–37 m/s with underlying zones containing fragmented and deformed WC grains. Such a structure provided plasticizing effect during sliding so that wear was mainly by flow of so plasticized subsurface layers.

1. Introduction

Sliding at high velocities is gaining more and more interest in tribology in connection with studying the tribological properties as well as both structural and tribochemical changes in new composite materials intended for high-speed sliding in railway, aviation brakes, and power gas turbine applications, especially in the search for phenomena that can be related to adaptation mechanisms [1,2]. These adaptation mechanisms are usually associated with the formation of antifriction and/or antiwear films on the worn surfaces by oxidizing the constituents of materials used in these tests by environmental oxygen [1,2,3,4,5,6]. The onset stage of such an adaptation mechanism can be severe subsurface deformation or grain refinement, generation of wear debris, their transfer, and then tribo-oxidation. The next stage will be reduction of adhesion, worn surface smoothing, and generation of continuous anti-friction films containing these mixed oxides.

Sliding at high-speeds usually means unlubricated sliding in the speed interval 5–90 m/s, and only few materials are capable of withstanding such severe conditions [7,8,9,10,11,12,13,14,15,16,17,18]. Those can be hybrid materials either fully composed of ceramics or containing ceramic or metal matrix composites, such as for example WC/Y–TZP–Al₂O₃ hybrid ceramic–matrix composites with dispersed Hadfield steel particles [7]. Sliding at 5–40 m/s speeds results in heating the samples to 1500–3000 °C and therefore, samples will be fully destroyed unless antifriction films are formed, which are capable of reducing the coefficient of friction [9,14,15].

Among numerous types of metal matrix composites, those sintered with high-manganese binder are widely used for fabricating drilling bits in mining and oil industries because of unique characteristics allowing combining their high hardness with high impact toughness [19,20,21,22]. In particular, their austenite matrix provides good retention of carbide grains under mechanical loading while impact loading may provoke partial martensitic transformation of the austenite that serves for higher wear resistance of the tool.

Tribological adaptation of W/Fe containing materials is usually associated with tribochemical synthesis of iron tungstates FeWO₄ and Fe₂WO₆, whose crystallographic lattice structure provides easy shear by corresponding planes and therefore allows demonstrating a self-lubrication effect [6,7,8]. Tribological in situ synthesis of FeWO₄ was discovered in sliding of WC–(Fe-Mn-C) composites on a steel disk at speeds of 7 to 37 m/s [8] and these results have then been used to create a low wear and low friction WC/Y-TZP-Al₂O₃ ceramics/cermet composite containing dispersed particles of high manganese steel.

Tribological experiments on sliding the electron beam composite M2 + WC coating have been carried out in the sliding speed range 0.8–3.6 m/s that resulted in simultaneous reduction in both wear rate and coefficient of friction [23]. It was demonstrated that such a finding was due to tribological adaptation driven by the tribochemical generation of lubricating FeWO₄ and Fe₂WO₆ mixed oxides on the worn surfaces.

Iron tungstates FeWO₄ and Fe₂WO₆ are of interest not only from the viewpoint of tribology. For instance, FeWO₄ is a promising electrode material for electrochemical capacitors working in neutral aqueous electrolytes [24]. Fe₂WO₆ is considered for use as a photocatalyst [25], photoelectrode material [26,27], or negative electrode for lithium-ion accumulators [28].

In this work, we focus on studying generation of a multilayer structure on earlier obtained and characterized WC/Mn13 steel cermets rubbed against a steel disk at speeds of 10 to 37 m/s. The aim is to elucidate the fundamental role of deformation, tribological transfer, and tribooxidation reactions of the elements composing the sliding bodies made of W- and Fe-containing materials

2. Materials and Methods

WC/Mn13 steel (WC/Hadfield steel) cermets were prepared using standard powder metallurgy methods, described in detail elsewhere [9]. Compressed green powder samples containing 70 wt.% WC and 30 wt.% Hadfield steel were free sintered at 1370 °C in vacuum, and then heated to 1100 °C with final quenching in the oil. The heat treatment conditions were chosen basing on the optimal ones earlier determined from the experiment and taking into account the quenching temperature used on WC/Mn13 steel cermet. The final sample dimensions were 5 × 5 × 8 mm³.

Such a choice of sintering modes and subsequent heat treatment made it possible to obtain hard metals with a binder phase represented by 60 vol.% of metastable austenite (γ–Fe phase) and 40 vol.% of α–Fe phase. The metallographic porosity of all WC/Mn13 steel cermet samples was about 6%. The WC grains had their mean size 2.8 ± 1.55 mm and were isolated from each other by 2.46 ± 1.21 μm binder interlayers (Figure 1a). The finally sintered composites had the following mechanical properties: compressive strength—3000 MPa, bending strength—1400 MPa, Vickers hardness—9500 MPa.

Microhardness numbers were obtained using a microhardness tester “Duramin-5” (Stuers A/S, Danemark) at 1 N indenting load.

A friction sliding testing machine UMT-1 (Tochpribor, Ivanovo, Russia) was used to carry out the testing. Cermet samples were prepared in the form of 5 × 5 × 8 mm³ pins intended for sliding over a disk made of as-cast high-vanadium high-speed steel (HSS) with hardness of 63–65 HRC. The HSS M2 steel contained phases such as martensite, M₁₂C precipitates, and a negligible fraction of retained austenite. Chemically, it was composed of elements as follows (wt.%): 15 V, 4.5 W, 3.6 Cr, 3.6 Mo, 1.8 Ni, 1.4 C, 1.0 Mn, and Fe-balance). Three pins were simultaneously secured in a circle at a diameter of 240 mm, pressed to the disk surface at nominal contact stress 5 MPa, and rubbed against it using sliding speeds of 10 m/s, 20 m/s, 30 m/s, and 37 m/s. The contact pressure was calculated by dividing the normal load 375 N by the total surface area of 75 mm². The environmental conditions were the following: temperature 25 °C and 30–40% humidity. Testing was carried out at fixed sliding speed thrice using a total of 9 pins. The wear path length was kept constant at 1000 m for each of the tests and this means that each test time was reduced in the order 100 s, 500 s, 33 s, and 27 s with increasing the sliding speed. To determine the wear rate, the sample’s height was measured before and after each testing, the volume loss calculated, averaged by three pins, and divided by the wear path length.

The phase detection was performed by means of Bragg–Brentano XRD and grazing incidence X-ray diffraction (GIAXD) geometries. The utilizing of both these methods was necessary for distinguishing between the phases in the tribological layers and the phases formed below the worn surface in the bulk of the WC/Mn13 steel cermet. A DRON-7 X-ray diffractometer with Co-Kα radiation, λ = 1.7902 Å (Burevestnik, Russia), was used for performing both XRD methods. The grazing-incidence X-ray diffraction was carried out with beam incidence angles of 10° and 5°, which provided the X-ray penetration depths ~1.2 μm and ~0.6 μm, respectively. The penetration values were calculated in accordance with [29]. Resulting XRD reflections were identified with the use of Crystal Impact’s software “Match!” (version 3.9, Crystal Impact, Bonn, Germany).

Sample were also examined with the help of following instruments: (1) scanning electron microscope (SEM) Philips SEM-515, an EDS add-on EDAX ECON IV (Koninklijke Philips Electronics N.V., Amsterdam, The Netherlands); (2) an SEM LEO EVO 50 (Carl Zeiss AG, Oberhochen, Germany), (3) Field emission SEM Tescan MIRA 3 LMU (TESCAN ORSAY HOLDING, Brno, Czech Republic), with an EDS detector (Oxford Instruments Ultim Max 40, Oxford Instruments, High Wycombe, UK).

3. Results

3.1. Wear Rate and Coefficient of Friction

The wear rate increased (Figure 2a) while the friction coefficient reduced from 0.15 to 0.07 (Figure 2b) as depending on the sliding speed increasing from 10 to 37 m/s.

3.2. Phases on the Worn Surfaces

XRD diffractograms obtained from the worn surfaces of WC/Mn13 steel cermet samples tested at all sliding speeds showed the XRD peaks being identified as those belonging to WC, γ–Fe, and FeWO₄ (Figure 3, Figure 4, Figure 5 and Figure 6). Starting from a speed of 20 m/s, the worn surfaces showed additional XRD peaks that may be identified as those belonging to Fe₃O₄ and α–Fe₂O₃ (Figure 4, Figure 5 and Figure 6).

Figure 3, Figure 4, Figure 5 and Figure 6 show that for worn surfaces obtained at all speeds and analyzed with the use of decreasing X-ray grazing angle, and, accordingly, the X-ray penetration, the intensities of peaks related to Fe₃O₄, α–Fe₂O₃, and FeWO₄ oxides grow with the sliding speed.

Semi-quantitative phase contents left on the worn surface were determined using the XRD peak intensities of FeWO₄ phases R(x) as calculated from Formula (1):

$$R(x)=I(x)/{\displaystyle \sum I(A)}$$

(1)

where I(x) is the intensity of the ‘x’ phase reflection related to the ΣI(A) sum of all reflection intensities.

It can be observed in Figure 7 that the amount of FeWO₄ is reduced as sliding speed increases. Moreover, the amount of FeWO₄ increases as the grazing angle reduces, i.e., reducing the X-ray penetration into the sample.

The specificity of oxide phases appeared after sliding at 10 and 20 m/s compared to those formed at 30 and 37 m/s is that the XRD peaks identified as those of FeWO₄ and WC allow suggesting that both these compounds were textured. In other words, the ratio of the integrated intensities is very different from those characteristics of these materials with spontaneous orientation of the crystallographic planes and demonstrated by powder X-ray diffraction patterns from the database for WC (code: 01-072-0097) and for FeWO₄ (code: 01-085-1354). The intensities of the same phases of FeWO₄ and WC found in the subsurface of samples tested at 30 and 37 m/s, in contrast, demonstrated good agreement with these tabular values. For WC, it is easy to visually assess this by comparing the ratio of peak intensities obtained from the worn surface (Figure 5 and Figure 6) and those from the polished composite surface after sintering (Figure 1c). They are almost identical and correlate well with the standard data.

The worn surfaces of the WC/Mn13 steel cermets resulted from rubbing at 10 and 20 m/s display bright and gray regions having their longest axes coinciding with the direction of sliding (Figure 8a,c). The brighter regions represent agglomerates of small WC fragments, which were formed and consolidated by rubbing against the steel disk. The gray regions represent oxidized steel binder areas of the composite with occasional inclusions of fine WC particles.

Upon sliding at 30 and 37 m/s, the WC/Mn13 steel cermets demonstrated the presence of a continuous tribolayer with a homogeneous micro-compositional structure with small WC fragments distributed in a dark gray oxidized matrix (Figure 8d–f). Compared to the worn surface at 10 and 20 m/s and the original composite, the worn surfaces obtained after sliding at 30 and 37 m/s are characterized by a higher content of iron (Figure 8f). After all sliding speeds, the EDS spectra allow detecting the presence of both vanadium and chromium on the worn surfaces (Figure 8b,f), i.e., elements that might have been transferred from the counterbody disk and then oxidized (Figure 1b).

The above-demonstrated data about generation of a homogeneous micro-composite layer at high sliding speeds (at 30 and 37 m/s) can be supplemented by observations of the Vickers pyramid indentations on the worn surface (Figure 9). The micro-composite structure of tribolayers generated after sliding at 10 and 20 m/s and composed of WC fragments and their agglomerates (Figure 9c–f) is clearly visible on the background of the original composite structures (Figure 9a,b). Almost homogeneous compositional structure observed in the depth of the indentations made through the worn surfaces resulted from rubbing at 30 and 37 m/s (Figure 9g,h).

It follows from observing plots in Figure 10 that that microhardness of the gray layer decreased with increasing the sliding speed still being below that of the as-sintered sample. Simultaneously, the microhardness of composite regions below this gray layer also reduced. The microhardness of these regions was higher than that of as-sintered sample after sliding at 10 m/s, while on sliding at 37 m/s, these microhardness numbers were almost identical.

It should be noted that when the composite was tested at speeds of 10 and 20 m/s, a lot of evidence of brittle fracture such as delamination of surface tribolayers (Figure 8a,c) [30] or cracks that propagate into the material to the depth of 15–30 µm (Figure 11a,b) can be observed.

No macro-cracks were observed on the tribological specimens after testing at 30 m/s and 37 m/s (Figure 8d,e). Evidence of brittle fracture was manifested as micro-cracking of the carbide grains at the scale of one grain (Figure 11c,d).

Examining the subsurface structures (Figure 12 and Figure 13) demonstrated generation of 3–5 μm-thick tribolayers on sliding at 10 and 20 m/s (Figure 12a–d) and 10–13 μm on sliding at 30 and 37 m/s. These tribolayers contain O, Fe, and W, as well as V and Cr, i.e., elements that have been transferred from the steel counterbody (Figure 12b,c;

Table 1. EDS elemental composition of subsurface area on WC/Mn13 steel cermet after sliding at 10 m/s (Figure 12b–d).

Area	Element (wt.%)
	O	V	Cr	Mn	Fe	W
Polished Cross Section
1	19.51	0.3	0.11	3.77	17.47	58.84
2	20.35	0.24	0.2	3.35	19.04	56.82
3	20.60	-	-	11.86	58.69	8.85
4	19.72	-	-	8.41	59.54	12.33
5	-	-	-	12.54	78.89	8.57
6	-	-	-	-	2.54	97.46
Fracture Cross Section
7	22.19	0.29	0.1	3.15	19.12	55.23
8	23.99	0.25	0.15	3.03	16.40	56.19
9	7.55	-	-	8.68	63.80	19.97
10	13.01	-	-	7.88	60.67	18.43
11	-			10.11	66.10	23.79
12	4.09	-	-	0.78	5.40	89.73
13	10.36	-	-	11.60	65.06	11.47
14	9.66	-	-	7.30	46.53	36.51
15	13.40	-	-	9.01	53.94	23.65
16	2.48	-	-	10.34	71.00	16.19
17	-	-	-	6.77	42.41	50.81
18	-	-	-	1.75	13.01	85.24

, areas 1, 2, 7, 8; Figure 13b,d;

Table 2. EDS chemical composition of subsurface area on WC/Mn13 steel cermet after sliding at 37 m/s (Figure 13b,d).

Area	Element (wt.%)
	O	V	Cr	Mn	Fe	W
Polished Cross Section
1	21.2	0.24	0.2	4.0	21.00	53.36
2	20.84	0.32	0.13	4.18	19.71	54.82
3	21.27	0.28	0.4	6.70	25.19	46.16
4	-	-	-	7.40	92.60
5					4.33	95.67
Fracture Cross Section
6	23.19	0.23	0.12	5.11	21.38	49.98
7	19.27	0.30	0.18	3.89	26.00	50.36
8	18.86	0.24	0.1	6.70	19.49	54.61
9	-	-	-	-	1.37	98.63
10	-	-	-	10.57	78.42	8.70
11	-	-	-	10.98	82.93	4.61

, areas 1, 2, 3, 6, 7, 8). Intensive oxidation was observed in the vicinity of cracks formed on sliding at 10–20 m/s (Figure 12b,c; Table 1, areas 3, 4, 9, 10) along with extended oxidized regions where no cracks are observed (Figure 12d; Table 1, areas 13–16). No such oxidizing was detected below the tribolayers after sliding at 30, 37 m/s (Figure 13b,d; Table 2, areas 4, 5, 9–11).

3.3. Wear Particles

The wear debris (Figure 14a and Figure 15a) are represented by particles of different morphological types: tiles (Figure 14c and Figure 15c) and unformed aggregates (Figure 14e and Figure 15e), which look as if they were composed of 0.1–10-μm-sized blobs. The EDS spectra allowed identifying in them the elements V and Cr along with O (Figure 14b,d,f and Figure 15b,d,f), i.e., elements transferred from the counterbody and then oxidized.

The EDS spectra of these wear particles may be compared with those obtained from the worn surfaces they have delaminated from. It could be concluded then that the wear particles contain higher oxygen concentrations. In addition, the concentration of tungsten is higher in the tile particles compared to that in the unformed aggregates, plausibly because the latter contain greater fraction of iron (Figure 14d,f and Figure 15d,f).

The size and percentage of wear particles of different morphology is sensitive to the sliding speed, as shown in Figure 16. As the sliding speed increases, the size of tile particles is decreased (Figure 16a,b) and more unformed aggregates are generated (Figure 16c,d).

Sectioned views of the wear debris allow us to observe internal voids in the irregular-shaped wear particles as well as different contrast between internal and periphery regions inside the unformed aggregates (Figure 17a–c and Figure 18) as well as their framework structures with bright inclusions identified as WC submicron-sized particles.

Generally, three different zones can be delineated in case of irregular-shaped wear particles, namely WC fragment agglomerates (Figure 17c;

Table 3. EDS chemical composition of the wear particles as tile fragments (Figure 17d) and irregular-shaped wear particle (Figure 17c). Wear particles were collected after sliding WC/Mn13 steel cermet at 20 m/s.

Area	Element (wt.%)
	O	V	Cr	Mn	Fe	W
Irregular Shaped Wear Particles
1	0.7	-	-	-	1.6	97.42
2	0.88	-	-	-	1.96	97.34
3	16.91	-	-	5.23	18.44	59.42
4	18.78	0.18	-	5.14	18.45	57.45
5	22.26	0.7	0.31	3.88	58.22	14.64
6	20.06	0.81	0.66	3.4	62.52	12.55
Wear Particles As Tile Fragments
7	1.2	-	-	-	2.4	96.4
8	0.62	-	-	-	1.71	97.67
9	19.52	0.32	0.56	5.5	38.27	36.19
10	19.29	0.41	0.74	4.99	45.28	29.05
11	4.23	0.1	-	0.99	20.33	74.35

, areas 1, 2), dark border zones enriched with iron and oxygen (Figure 17c; Table 3, areas 5, 6, Figure 18e;

Table 4. EDS chemical composition of the wear particles as tile fragments (Figure 18e) and irregular-shaped wear particle (Figure 18c). Wear particles were collected after sliding WC/Mn13 steel cermet at 37 m/s.

Area	Element (wt.%)
	O	V	Cr	Mn	Fe	W
Irregular Shaped Wear Particles
1	35.13	0.24	-	0.4	70.95	3.29
2	25.63	-	-	0.81	71	2.55
3	19.88	1.95	0.39	1.59	27.01	48.47
4	18.18	0.36	-	1.15	27.66	52.64
Wear Particles As Tile Fragments
5	17.15	0.46	0.28	6.72	24.35	50.87
6	16.23	0.37	0.37	4.68	26.1	51.53
7	1.17	-	-	-	2.9	95.94
8	1.35	-	-	-	2.08	96.58

, areas 1, 2), and bright-gray zones containing O, W, and Fe in concentrations that allow suggesting the formation of FeWO₄ (Figure 17c; Table 3, areas 3, 4; Figure 18e; Table 4, areas 3, 4).

The tile wear particles are characterized by the presence of two zones, the WC agglomerates (Figure 17d; Table 3, areas 7, 8; Figure 18e; Table 4, areas 7, 8) and zones with almost equal contents of WC and oxidized matrix (Figure 17d; Table 3, area 9, 10; Figure 18e; Table 4, area 5, 6).

4. Discussion

4.1. In Situ Tribosynthesized FeWO₄

Increasing the sliding speeds to 30 and 37 m/s caused intensive heating of the WC/Mn13 steel cermet samples, which even produced intensive red-yellow glow. However, an infrared pyrometer showed the temperature of ~600 °C as measured in the point located on the lateral face of the pin 1–3 mm beneath the worn surface (Figure 19a).

Temperature in the sliding contact zone can be evaluated by calculating the flash temperature from the equation [31]:

$$∆T=\frac{\mu PV}{4J\left(K_{sample}+K_{disk}\right)\mathsf{\alpha }}$$

(2)

where μ is the coefficient of friction, P and V are the load and speed of sliding, respectively, and K_sample and K_disk are the heat conductivities of the sample and counterbody, respectively. J is the Joule’s constant (J = 1) and α is the real contact area that could be obtained from the following Equation (4):

$$\alpha ={\left(\frac{P}{\pi H_{sample}}\right)}^{1/2}$$

(3)

$H_{sample}$ is the hardness of the cermet.

The flash temperature was estimated starting from the test parameters as follows: P = 375 N, hardness 9.5 GPa, V = 10–37 m/s, and µ = 0.16–0.07 and as a function of sliding speed (Figure 3b). Heat conductivity values of the WC/Mn13 steel cermet and HSS M2 disk were assumed as 60–80 W⋅m⁻¹ K⁻¹ and 25–40 W⋅m⁻¹ K⁻¹, respectively, according to [32,33,34]. Figure 17b shows how the calculated flash temperatures increase from 290 to 400 °C.

It is common that high temperature flashes appear on the real contact areas, after which the heat sinks into the bulk of the sample, thus increasing its average temperature where it may be measured. As a rule, flashpoint temperatures are several times higher than those measured by a pyrometer [7,8]. In our case, the temperatures measured with the pyrometer were higher than those calculated from Equation (2) (Figure 19b). It is known [35] that the maximum temperature in sliding appears at some distance below the worn surface in certain materials, especially in those with high thermal conductivity.

It is known [36] that under equilibrium conditions, oxidation of WC to WO₃ may start at temperatures of ~800 °C. In the case of high-temperature sliding and deformation, this reaction may start at even lower temperatures because of grain refining and the effect of attrition. In terms of tribological adaptation, the positive effect of such a tribooxidation may be the intensification of FeWO₄ synthesis from both iron and tungsten oxides starting from temperatures in the range ~400–600 °C [37].

The probabilities of chemical Reactions (4)–(13) occurring in sliding was evaluated by calculating corresponding free energy values.

2Fe + O₂ = 2FeO

(4)

4/3Fe + O₂ = 2/3Fe₂O₃

(5)

FeO + Fe₂O₃ = Fe₃O₄

(6)

3/2Fe + O₂ = 1/2Fe₃O₄

(7)

4FeO + O₂ = 2Fe₂O₃

(8)

WC + 2O₂ = WO₃ + CO

(9)

WC + 5/2O₂ = WO₃ + CO₂

(10)

WC + Fe₃O₄ = WO₃ + Fe + CO

(11)

FeO + WO₃ = FeWO₄

(12)

1/3Fe₂O₃ + 1/3Fe + WO₃ = FeWO₄

(13)

All these reactions can be classified into three types: iron oxidation Reactions (4)–(6), tungsten carbide oxidation (9)–(11), and synthesis of FeWO₄ (12), (13). The free energy calculations were carried out for iron and tungsten oxides per 1 mole of oxygen and tungsten oxide, respectively. The probability of occurrence of Reactions (4)–(13) was evaluated by free energy changes ΔG = ΔH – T × ΔS in the temperature range 543.15–1173.15 K (270–900 °C) using the thermodynamic characteristics of Fe [38], O₂ [39], FeO [40], Fe₂O₃ [40], Fe₃O₄ [40], WC [38], WO₃ [41], CO [39], CO₂ [39], and FeWO₄ [38]. The calculated Gibbs energy values for Reactions (4)–(13) are shown in

Table 5. Gibbs free energy changes for reactions (4–138) vs. temperature.

Reaction	ΔG, kJ/mol
t, °C	270	460	520	530	600	700	800	900
T, K	543.15	733.15	793.15	803.15	873.15	973.15	1073.15	1173.15
4	−469.00	−442.73	−434.44	−433.06	−423.38	−409.55	−395.73	−381.90
5	−450.78	−415.96	−404.96	−403.13	−390.30	−371.97	−353.64	−335.32
6	−21.78	−21.23	−21.05	−21.03	−20.82	−20.53	−20.25	−19.96
7	−466.23	−433.27	−422.86	−421.12	−408.98	−391.64	−374.29	−356.94
8	−414.36	−362.42	−346.02	−343.28	−324.15	−296.81	−269.48	−242.14
9	−819.73	−787.16	−776.87	−775.16	−763.16	−746.01	−728.87	−711.73
10	−1055.76	−1006.76	−991.29	−988.71	−970.66	−944.87	−919.08	−893.30
11	112.72	79.38	68.85	67.09	54.81	37.26	19.71	2.16
12	−65.10	−64.13	−63.83	−63.78	−63.43	−62.92	−62.41	−61.91
13	−74.21	−77.52	−78.57	−78.74	−79.97	−81.72	−83.45	−85.21

and Figure 20, Figure 21 and Figure 22.

As follows from dependencies in Figure 20, Reactions (4), (7), (5), and (8) are more likely to occur as listed in descending order. Reaction (6) is less probable in this temperature range. It should be noted that the probability of iron oxide reaction occurrence decreases as the temperature increases.

Reactions (6)–(8) (Figure 21) require free energy changes per 1 mole of WC for the formation of WO₃. It is easy to see that the WC oxidizing is more likely to occur according to Reactions (9) and (10).

The Gibbs free energy dependencies on temperature for Reactions (12) and (13) allow observing that the probability of FeWO₄ synthesis according to Reaction (12) is linearly reduced with the temperature (Figure 21), while it is more likely to synthesize FeWO₄ according to Reaction (13) when the higher the temperature, the less the free energy change.

The amount of FeWO₄ on the worn surfaces has a tendency to reduce with increasing sliding speed (Figure 7) along with the presence of iron oxides Fe₃O₄ and Fe₂O₃ (Figure 4, Figure 5 and Figure 6). Therefore, one may suggest that Reaction (12) is the most probable in our case, being preceded by Reaction (4) which gives FeO as a product. In addition to those giving FeO and FeWO₄, others that allow producing Fe₃O₄ and Fe₂O₃ such as Reactions (7) and (8) may occur. It is worth noting that the WO₃ is more likely to form in Reaction (11) between WC and Fe₃O₄ (Figure 21, Table 5) than in direct tribooxidizing the WC by oxygen (Reaction (10), Figure 21, Table 5).

4.2. Wear Mechanism

Some explanations are provided below for the coefficient of friction reducing but wear rate increasing with the sliding speed. The wear of all metal matrix composites reinforced with hard particles is by pulling out the particles due to adhesion and scratching the counterbody. The same is true with a WC/Mn13 steel cermet [42,43,44,45,46] when ductile austenite steel binder is pulled out of the interparticle spaces by adhesion and then nothing prevents the carbide grains from pulling out and scratching the worn surfaces [42,43,44]. The wear grooves and pits left by the particles may also serve as stress concentrators suitable for subsurface crack nucleation [44,47,48]. The pulled-out WC grains scratch the worn surface and increase the contact stresses, thus causing more severe plastic deformation of the matrix and fragmentation of these WC grains l [45,49,50]. Adhesion interaction between two bodies in rubbing will cause the intermixing and transfer of wear debris so that WC fine fragments may intermix with the plasticized transferred steel and thus form agglomerates [51,52,53].

Frictional heating and attrition can easily oxidize the wear debris according to Reactions (4), (7), (8), (11), and (12). The WC agglomerates may oxidize outside of the contact zone and form irregular-shaped wear particles (Figure 14e, Figure 15e, Figure 17c and Figure 18c) while another part of them is smeared over the worn surfaces to form tribolayers composed of WC fragments and plasticized metal as occurs in case of sliding at 10 and 20 m/s (Figure 8c and Figure 9c,e). The tribooxidized agglomerates may also form densified mushy layers that fully cover the nominal contact area in the form of mechanically mixed layer (Figure 12 and Figure 13) [54,55,56,57,58]. On sliding at 10 and 20 m/s, these layers provide reduction in friction while wear is by the delamination of thin tile wear particles whose thickness is comparable to that of the mechanically mixed layer (Figure 10) [51,59,60,61,62,63].

Tribological studies have shown that the coefficient of friction of WC/Mn13 steel cermet rubbed with sliding speed of 37 m/s against the steel disk dropped to very low values (~0.07, Figure 2b). It is logical to suggest that this behavior is due to tribological adaptation associated with the tribochemical generation of a self-healing subsurface protective layer consisting of small WC fragments and wear products containing FeWO₄ complex oxide synthesized in the process of high-speed sliding (Figure 5 and Figure 6), which is prone to performing self-lubricity due to its low shear strength [6]. The composites under consideration are intensively heated during sliding at 37 m/s, which contributes to thermal plasticization and transfer of high-speed steel components from the steel disk to the composite, as well as the preferential extraction of components out of the composite steel matrix to the sliding contact zone. Sliding WC/Mn13 steel cermet at a speed of 30 and 37 m/s and contact pressure of 5 MPa over the HSS steel counterbody resulted in the generation of a continuous mechanically mixed and oxidized layers (MML) consisting of small tungsten carbides with iron oxides enveloped by mechanochemically synthetized FeWO₄ (Figure 5, Figure 6, Figure 8d,e, and Figure 13a,b).

The MML generated on sliding at 30 m/s and 37 m/s may serve as efficient anti-friction coating that shields the underlying material from intense oxidation (Figure 13b,d; Table 2, areas 4, 5, 9–11) and fragmentation (Figure 11c,d).

Additional investigation into structural changes occurring below the worn surfaces was carried out using a layer-by-layer XRD with sequential grinding away 10–20 µm thick layers. The result was a set of in-depth dependencies characterizing changes of the WC-grain coherent areas’ size (CAS). Figure 23a–d shows that for all sliding speeds, CAS is varied with the distance below the worn surface as a curve with a maximum. The location of the maximum of these dependences is determined by the sliding speed and, as can be seen in Figure 23e, this position of the CAS maximum is at the maximum distance below the surface for the sample rubbed against the steel at 20 m/s. At the same time, for all sliding speeds, the maximum WC CAS value exceeds all these values obtained from the polished surface of the original composite after sintering. Apparently, at a distance of 70 µm to 230 µm, depending on the sliding speed, the area of the material under the friction surface is subjected to intensive temperature effect, which anneals internal defects and contributes to increasing the WC CAS values.

Nevertheless, more intensive frictional heating (plasticizing) and adhesion pulling out the matrix material occurs in sliding at 30 and 37 m/s as compared to those at 10 and 20 m/s facilitating the loss of the composite load bearing capacity (Figure 10) and enhanced wear by flow (Figure 2a).

Table 6. Overview of the wear results and mechanisms for different speeds.

Speed	10, 20 m/s	30, 37 m/s
Wear surface	Deep abrasive grooves, surface cracks, tile wear particles. Tribolayers with a micron-sized agglomerates of fine particles WC.	Smooth surfaces coated with a continuous and defect-free tribolayer with a homogeneous micro-composite structure
Wear subsurface	Paste-like thin MML formation with delaminated fragments. Cracks connected to the delaminated fragments.	Continuous, defect-free paste-like MML formation with a homogeneous micro-composite structure. Minor damage on the scale of one or two carbide grains below tribolayer.
Wear debris	Tile wear particles of the size 160–190 μm. Irregular-shaped wear particles of the size 120–140 μm.	Tile wear particles of the size 110–130 μm. Irregular-shaped wear particles of the size 80–100 μm.
Wear rate	0.01 mm3/m, 0.24 mm3/m	0.39 mm3/m, 0.46 mm3/m
Average friction coefficient	0.17, 0.15	0.10, 0.07
Wear mechanism	MML delamination	Flow wear

serves to summarize the results obtained in this work, including the worn surface morphology and wear rate.

5. Conclusions

WC-Mn13 (Hadfield steel) composites with 70 wt.% WC were prepared using the powder metallurgy free sintering method. Phase composition, microstructure, friction, and wear properties were studied in sliding the composite on a high-speed steel disc at 10, 20, 30, and 37 m/s.

It was established that increasing the sliding speed from 10 to 37 m/s resulted in increasing the wear rate with simultaneous reduction of the coefficient of friction. The reason behind friction reduction was the generation of a tribological layer (MML) composed of oxidized WC wear fragments, iron oxides, and mixed oxide FeWO₄, whose total amount on the worn surface decreased with increasing the sliding speed. It was shown that the amount of in situ generated FeWO₄ reduced with the distance below the worn surface.

The sliding speed had an effect on the subsurface structure and tribolayer thickness so micron-sized, mechanically mixed 3–4 μm-thick layers were generated in sliding at 10 and 20 m/s, whose wear occurred mainly by subsurface fracture and delamination of tile wear particles.

Continuous tribolayers with 10–15 μm thickness were generated at 30–37 m/s with underlying zones containing fragmented and deformed WC grains. Such a structure provided a plasticizing effect during sliding so that wear was mainly by flow.

Friction reduction with the sliding speed was related to the phenomenon of tribological adaptation by the generation a self-healing micro-sized composite protective plasticized layer composed of small WC particles, iron oxides enveloped by in situ produced FeWO₄.