Sliding wear characteristics of gas boronized steel

The wear characteristics and the mechanism of sliding wear of boronized steel under unlubricated conditions were studied. Characteristic wear curves of FeB and Fe₂B boride layers formed on SAE 1045 steel were similar in form. The maximum wear rates were obtained under a sliding velocity of 0.30 m s^?1 for FeB specimens and 0.50 m s^?1 for Fe₂B specimens. Under such conditions both mechanical wear caused by scratching and oxidative wear occurred. Under conditions of mild wear the wear loss was caused mainly by oxidative wear. Under conditions of heavy wear destruction of the sliding surface was caused by thermal stress. The wear debris was composed principally of iron oxides (α Fe₂O₃, Fe₃O₄) formed by oxidative wear, α iron and borides (FeB, Fe₂B) produced by mechanical wear and B₂O₃ produced by the preferential oxidation of boron in the boride layer.     

The sliding wear of 316 stainless steel in air in the temperature range 20–500°C

The friction and reciprocating wear of 316 stainless steel in air has been investigated in the temperature range 20–500°C at constant load using a standard pin and flat geometry. A marked change in wear behaviour occurred above 300°C. From room temperature to 300°C the wear rate decreased slowly with increasing temperature. This was accompanied by an increasing fraction of oxide in the wear debris. At 300°C the debris consisted entirely of oxide with the α Fe₂ O₃ structure. In this temperature range wear can be explained essentially in terms of mild wear. Above 300°C the wear rate decreased by an order of magnitude and was accompanied by a severely distorted wear surface. There was a high proportion of metallic particles in the wear debris. The surface roughening occurs at an early stage of wear and stops when glazed oxide regions form. The low wear rate is explained in terms of the high hardness of the glazed load-bearing areas and re-incorporation of wear debris into the wear scar.     

Comparison of the tribological behavior of columnar-grained polycrystalline copper with vertical/horizontal orientation sliding against AISI 1045 steel

The unlubricated tribological behavior of columnar-grained polycrystalline copper with vertical orientation (VO) and horizontal orientation (HO) in sliding against AISI 1045 steel is investigated in room air. The friction coefficient and wear rate of HO Cu in sliding against AISI 1045 steel are lower than that of the VO Cu. The friction-induced layers are distinct. The friction-induced layer on HO Cu is composed of metallic Cu, Fe₂O₃, and Fe₃O₄. The wear debris is agglomerated fine particles. The friction-induced layer on VO Cu presents characteristics of plastic deformation and flow. The wear debris is plate-like. The grain orientations and boundaries of VO/HO Cu sample are contributed to the different friction-induced layer and wear mechanism. For VO Cu, the grain boundary perpendicular to the sliding direction makes the slip motion activate difficultly. For HO Cu, the grain boundary parallel to the sliding direction makes the slip motion activate easily.     

Effects of supply of fine oxide particles onto rubbing steel surfaces on severe–mild wear transition and oxide film formation

This study is the first to show a quantitative condition required for the establishment of severe–mild wear transition with sliding distance, by studying the effects of supply of Fe₂O₃ particles onto rubbing steel surfaces on the transition and oxide film formation process. The supply of fine Fe₂O₃ particles was found to accelerate the wear transition, and the sliding distance at which the transition occurs was found to increase with particle diameter and applied load. Oxide films are produced on the rubbing surfaces by sintering of the supplied Fe₂O₃ oxide particles. At the severe–mild wear transition, the relative area of oxide films is the same for all diameters of supplied Fe₂O₃ particles. This finding suggests that the transition occurs when the relative area of oxide films reaches a specific value, which is proportional to the area of real contact.     

Wear studies on boronized mild steel

Unlubricated dry sliding wear tests were carried out using a pin-on-disc machine on boronized mild steel pins containing FeB, Fe₂B and both phases in the surface layers. The discs were made of hardened tool steel. Singlephase boride layers exhibited better wear resistance than two-phase boride layers. The wear debris contained essentially Fe₂B, α-Fe, Fe₃O₄ and Fe₂O₃. The untreated mild steel substrates and the FeB layers undergo oxidation and mechanical abrasion during wear tests. The two-phase borides show adhesive transfer and oxidation. The Fe₂B layers fail by fatigue.     

Wear and Friction Characteristics of a Selected Stainless Steel

Dry sliding wear tests were performed for 3Cr13 steel with various tempered states at 25–400°C; wear and friction characteristics as well as the wear mechanism were explored. With an increase in test temperature, the wear rate decreased accompanied by an increase in tribo-oxides. The fluctuation of friction coefficient was slight at 25–200°C but became violent at 400°C. At 25–200°C, adhesive wear prevailed due to trace or less tribo-oxides; at 400°C, oxidative wear prevailed with the predominant tribo-oxides of Fe₃O₄ and Fe₂O₃. It can be suggested that the antioxidation of the stainless steel postponed the occurrence of oxidative wear to a higher test temperature. For adhesive wear, the wear resistance, roughly following Archard's rule, was directly proportional to hardness besides the specimen tempered at 500°C with grain boundary brittleness. But for elevated-temperature wear, a better wear resistance required thermal stability and an appropriate combination of hardness and toughness.     

Mild wear of a low alloy steel at temperatures up to 500°C

Wear behaviour of 52100 low alloy steel has been studied on a pin on disc wear machine at disc temperatures ranging from room temperature to 500°C. Transitions occur in the wear rate versus load curves at certain critical loads, the magnitude of which increase with temperature. These transitions were found to be associated with change in surface oxide, lower wear rates being recorded when the predominant oxide was the spinel Fe₃O₄ for all temperatures. At disc temperatures above 300°C out of contact oxidation appears to be the most important wear limiting factor. A surface model was developed enabling contact temperature, numbers and size of contacts and critical oxide film thickness to be deduced. Remarkable agreement was found between oxide thicknesses estimated from this model and measured values using a scanning electron microscope     

Tribofilm formation and mild wear by tribo-sintering of nanometer-sized oxide particles on rubbing steel surfaces

The present study is the first to show that the supply of nanometer-sized particles of Fe₂O₃, SnO₂, CuO, or Bi₂O₃ oxide on rubbing steel surfaces induces transition to mild wear with sliding distance, and that the wear transition behavior depends on the type of supplied oxide. The mild wear is due to formation of the wear-protective tribofilm on the rubbing surfaces, and observations confirm that the tribofilms are produced by tribo-sintering of the supplied oxide particles. The mild wear transition behavior is explained by the sintering rate of the supplied oxide particles, which is related to the oxygen diffusion coefficient in the oxide and the particle diameter. When the supplied oxide is of high diffusivity, the tribofilm formation rate is high, owing to the high sintering rate of the oxide particles, and the mild wear transition occurs at a short sliding distance. In the case of Fe₂O₃ oxide, the sliding distance of the transition from severe to mild wear is decreased when finer particles are supplied, suggesting that fine particles are easily sintered on the wear surface.     

Transition of Mild Wear to Severe Wear in Oxidative Wear of H21 Steel

Under atmospheric conditions at 400 °C, we studied the wear mechanism of H21 steel with different tempering states as a function of normal load. Typical oxidative wear was identified by X-ray diffraction patterns with predominant tribo-oxides of Fe₃O₄ and Fe₂O₃. Under loads of 50–100 N, mild oxidative wear prevailed for all samples, such that the wear losses of H21 steel with various tempering states showed no significant differences with characteristics of a slight plastic deformation of the substrate and single-layer oxide. In this case, the wear rate was lower, and the tribo-oxide was decisive factor in determining wear rate. Under loads of 150–200 N, the transition of mild wear to severe wear occurred in H21 steel and was characterized by: (1) a significant difference of wear losses for steel with various tempering states; (2) wear loss that started to increase faster and reached a relatively high level; (3) the appearance of significant plastic deformation in the oxide underneath the substrate and multi-layer tribo-oxide. Under a load of 200 N for the steel tempered at 700 °C, plastic extrusion prevailed with a mixed metal-oxide layer.     

Abrasive wear behavior of boronized AISI 8620 steel

In this study, AISI 8620 steel was boronized using the solid state boronizing technique. Processes were carried out at the temperatures of 850, 900 and 950 °C for 2, 4 and 6 h of treatment. Abrasive wear behavior of the samples boronized at different temperatures and treatment durations have been examined. Using boronized and unboronized samples, abrasive tests were conducted using pin on disc test apparatus. 80 and 120 mesh aluminum oxide (Al₂O₃) abrasive papers were used in the abrasion experiments and the samples were subjected to abrasion under 10, 20 and 30 N loads. Boronized steels exhibited an improvement in abrasive wear resistance reaching up to 500%. Microstructures and wear surfaces of the samples were inspected using SEM microscopy. SEM examinations revealed that the thickness of the boride layer on the steel surfaces changes with changing process durations and temperatures. The presence of boride formed in the borided layer at the surface of the steels were determined by XRD analysis and microhardness values of the iron borides (FeB, Fe₂B) formed on the steel surface were found to be over 1600 HV.     

Effect of niobium interlayer on high-temperature sliding friction and wear of silver films on alumina

In this study, we investigated the effect of a thin Nb bonding layer (15–20 nm thick) on the high-temperature sliding friction and wear performance of Ag films ( 1.5 m thick) produced on -alumina (Al₂O₃) substrates by ion-beam assisted deposition (IBAD). The friction coefficients of Al₂O₃ balls against the Ag-coated Al₂O₃ flats were 0.32 to 0.5 as opposed to 0.8 to 1.1 against the uncoated flats. Furthermore, these Ag films reduced the wear rates of Al₂O₃ balls by factors of 25 to 2000, depending on test temperature. Wear of Ag-coated Al₂O₃ flats was hard to measure after tests at temperatures up to 400°C. At much higher temperatures (e.g., 600°C), these Ag films (without a Nb layer) were removed from the sliding surfaces and lost their effectiveness; however, Ag films with the Nb bonding layer remained intact on the sliding surfaces of the Al₂O₃ substrates even at 600°C and continued to impart low friction and low wear.     

Abrasive and Adhesive Wear Behavior of Arc-Evaporated Al<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N Hard Coatings

During the last decade, the usage of difficult-to-machine materials such as austenitic stainless steels has increased continuously in various industrial applications. Tools such as blind hole taps, punches, or deep drawing molds are often exposed to severe wear while machining/forming these materials, mainly due to excessive adhesion and material transfer. On combination with abrasive wear due to work-hardened wear debris, tool lifetime in these applications is often limited. In this study, ball-on-disc experiments were carried out with arc-evaporated AlCrN coatings with different Al/(Al + Cr) ratios against Al₂O₃ and austenitic stainless steel balls in ambient atmosphere. Test temperatures of 25, 500, and 700°C were chosen for the hard Al₂O₃ balls simulating severe abrasive loads, whereas 25, 150, and 250°C were used for the softer stainless steel material to evaluate the adhesive wear behavior. Characterization of the wear tracks was done by scanning electron microscopy in combination with energy-dispersive X-ray analysis and optical profilometry. The best abrasive wear resistance during testing against Al₂O₃ was observed for the coating with the highest Al content. In the case of the austenitic stainless steel balls, sticking of the ball material to the coating surface was the dominating wear mechanism. The influence of test temperature, chemical composition, and surface roughness was studied in detail.     

Reciprocating sliding wear of 9% Cr steel in carbon dioxide at elevated temperatures

Experiments were conducted on the initial stages of reciprocating sliding wear of a 9% chromium steel in an environment of carbon dioxide at temperatures in the range 200 to 550°C. At ambient temperatures of 290°C and above, an initial severe wear mode was followed by a transition to mild oxidational wear. At any given ambient temperature above 290°C, the distance of sliding required to reach such a transition was found to depend on load and mean sliding speed, although the dependency on speed was not simple. When a transition occurred, most of the surfaces were covered with a stable oxide film which consisted of an agglomerate layer of wear debris being mainly of oxide at the surface and mainly at the metal boundary. This film was supported by a work hardened layer extending for about 30 μm into the bulk of the metal. A surface model is proposed to explain the mechanism of formation of the supportive oxide layer; predictions of volume of material removed and final oxide coverage at the transition are in close agreement with experimental values     

Tribological Properties of Double-Glow Plasma Surface Niobizing on Low-Carbon Steel

The niobized layer was formed on Q235 low-carbon steel by double-glow plasma surface niobizing to improve its wear resistance. The microstructure, phase composition, and microhardness were determined. The friction and wear properties of the niobized samples and the untreated alloys were tested on a ball-on-disk tribometer by rubbing against GCr15 and silicon nitride (Si₃N₄) balls at room temperature and 400°C, respectively. The results indicated that the alloyed layer that contained a sediment layer and diffusion layer is about 35 μm in thickness, metallurgically adhered to the base metal. Niobium content was gradually decreased along the depth direction from the surface, which was similar to the change in the microhardness. The alloying layer mainly consisted of Nb, Fe₂Nb, and FeNb phases. Under unlubricated sliding conditions, the friction coefficients and the specific wear rates were lower than those of the untreated carbon steel at room and high temperatures. The wear mechanism of the niobized specimen at room temperature is dominated by slightly abrasive wear, whereas the predominant wear mechanism is abrasive wear and fatigue delamination at high temperature.     

Severe-to-Mild Wear Transition of Titanium Alloys as a Function of Temperature

Dry sliding wear behaviors of Ti–6Al–4V and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloys (code-named TC4 and TC11, respectively) against AISI 52100 steel under a load of 50–250 N at 25–600 °C were systematically investigated. For two titanium alloys, a severe-to-mild wear transition occurred with an increase in temperature. The critical transition temperatures of TC4 and TC11 alloys were 400 and 300 °C, respectively. Below the critical temperature, titanium alloys showed poor wear performance. As the temperature surpassed the critical temperature, the extremely low wear rates demonstrated excellent elevated-temperature wear performance of titanium alloys in the titanium alloy/steel tribo-system. The wear transition was characterized with the appearance of continuous, hard tribo-layer containing more oxides, especially Fe₂O₃, which showed a pronounced wear-reduced role. Adhesive and abrasive wear predominated in the severe wear regime; oxidative mild wear prevailed in the mild wear regime. Adhesive wear, abrasive wear and oxidative mild wear cooperated at the critical transition temperatures.     

Friction, wear and material transfer of sintered polyimides sliding against various steel and diamond-like carbon coated surfaces

Polyimide cylinders are slid under 50 N normal load and 0.3 m/s sliding velocity against carbon steel (R_a=0.2 and 0.05 μm), high-alloy steel (R_a=0.05 μm), diamond-like carbon (DLC, R_a=0.05 μm) and diamond-like nanocomposite (DLN, R_a=0.05 μm). Only for a limited range of test parameters, the friction of polyimide/DLN is lower than for polyimide/steel, while polyimide shows higher wear rates after sliding against DLN compared to steel counterfaces. The DLN coating shows slight wear scratches, although less severe than on DLC-coatings that are worn through thermal degradation. Therefore, also friction against DLC-coatings is high and unstable. Calculated bulk temperatures for steel and DLN under mild sliding conditions remain below the polyimide transition temperature of 180 °C so that other surface characteristics explain low friction on DLN counterfaces, as surface energy, structural compatibility and transfer behaviour. Friction is initially determined through adhesion and it is demonstrated that higher surface energy provides higher friction. After certain sliding time, different polyimide transfer on each counterface governs the tribological performance. Polyimide and amorphous DLC structures are characterised by C–C bonds, showing high structural compatibility and easy adherence of wear debris on the coating. However, it consists of plate-like transfer particles that act as abrasives and deteriorate the polyimide wear resistance. In sliding experiments with high-alloy steel, wear debris is washed out of the contact zone without formation of a transfer film. Transfer consists of island-like particles for smooth carbon steel and it forms a more homogeneous transfer film on rough carbon steel. The latter thick and protective film is favourable for low wear rates; however, it causes higher friction than smooth counterfaces.     

Synthesis and evaluation of mechanical and high temperature tribological properties of in-situ Al-TiC composites

In situ Al-TiC (5, 10 and 15 wt%) composites were produced by using a reaction mixture of K₂TiF₆ and graphite powder with molten metal. The effect of ceramic particulate addition on the high temperature sliding wear resistance of the composites was studied. The sliding wear tests were conducted at room temperature, 120 and 200 °C. The wear rate increases with the increase in applied load and decreases with increase in the weight percentage of TiC. Both monolithic and composites were able to withstand thermal softening effects due to the formation of oxidative protective transfer layer.     

Tribological characteristics of low-pressure plasma-sprayed A12O3 coating from room temperature to 800 °C

The tribological characteristics of low-pressure plasma-sprayed (LPPS) Al₂O₃ coating sliding against alumina ball have been investigated from room temperature to 800 °C. These friction and wear data have been compared quantitatively with those of bulk sintered alumina to obtain a better understanding of wear mechanisms at elevated temperatures. The friction and wear of Al₂O₃ coating show a strong dependence on temperature, changing from a mild to a severe wear regime with the increase of temperature. The coefficient of friction at room temperature is approximately 0.17 to 0.42, depending on applied load. The tribochemical reaction between the coating surface and water vapor in the environment and the presence of the hydroxide film on the Al₂O₃ coating reduce the friction and wear at room temperature as contrasted to those of bulk sintered alumina. At intermediate temperatures, from 400 to 600 °C, the friction and wear behavior of Al₂O₃ coating depends on the inter-granular fracture and pull-out of Al₂O₃ grains. At above 700 °C, formation and deformation of fine grain layer, and abrasive wear in the form of removal of fine alumina grains further facilitate the friction and wear process of Al₂O₃ coating.     

Friction and wear behaviour of plasma-sprayed Cr2O3 coatings against steel in a wide range of sliding velocities and normal loads

This study investigates the influence of sliding speed and normal load on the friction and wear of plasma-sprayed Cr₂O₃ coatings, in dry and lubricated sliding against AISI D2 steel. Friction and wear tests were performed in a wide speed range of 0.125–8 m/s under different normal loads using a block-on-ring tribometer. SEM, EDS and XPS were employed to identify the mechanical and chemical changes on the worn surfaces. A tangential impact wear model was proposed to explain the steep rising of wear from the minimum wear to the maximum wear. The results show that the wear of Cr₂O₃ coatings increases with increasing load. Secondly, there exist a minimum-wear sliding speed (0.5 m/s) and a maximum-wear sliding speed (3 m/s) for a Cr₂O₃ coating in dry sliding. With the increase of speed, the wear of a Cr₂O₃ coating decreases in the range 0.125–0.5 m/s, then rises steeply from 0.5 m/s to 3 m/s, followed by a decrease thereafter. The large variation of wear with respect to speed can be explained by stick-slip at low speeds, the tangential impact effect at median speeds and the softening effect of flash temperature at high speeds. Thirdly, the chemical compositions of the transfer film are a-Fe₂O₃ in the speed range 0.25–2 m/s, and FeO at 7 m/s. In addition, the wear mechanisms of a Cr₂O₃ coating in dry sliding versus AISI D2 steel are adhesion at low speeds, brittle fracture at median speeds and a mixture of abrasion and brittle fracture at high speeds. Finally the lubricated wear of Cr₂O₃ coating increases sharply from 1 to 2.8 m/s.     

TEM and AES investigations of the natural surface nano‐oxide layer of an AISI 316L stainless steel microfibre

The chemical composition, nanostructure and electronic structure of nanosized oxide scales naturally formed on the surface of AISI 316L stainless steel microfibres used for strengthening of composite materials have been characterised using a combination of scanning and transmission electron microscopy with energy‐dispersive X‐ray, electron energy loss and Auger spectroscopy. The analysis reveals the presence of three sublayers within the total surface oxide scale of 5.0–6.7 nm thick: an outer oxide layer rich in a mixture of FeO.Fe₂O₃, an intermediate layer rich in Cr₂O₃ with a mixture of FeO.Fe₂O₃ and an inner oxide layer rich in nickel.