The etching behaviour of silicon carbide compacts

A series of microstructural investigations has been undertaken in order to explore the reliability of particular etches in revealing microstructural detail in silicon carbide compacts. A series of specimens has been etched and examined following complete prior microstructural characterization by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffractometry techniques. In particular, the sensitivity of both a molten salt (KOH/KNO₃) etch and a commonly-used oxidizing electrolytic ‘colour’ etch to crystal purity, crystallographic orientation and polytypic structure has been established. The molten salt etch was found to be sensitive to grain boundaries and stacking disorder while the electrolytic etch was found to be primarily sensitive to local purity and crystallographic orientation. Neither etch appeared intrinsically polytype sensitive. Specifically, for the ‘colour’ etch, the p- or n-type character of impure regions appears critical in controlling etching behaviour; p-type impurities inhibiting, and n-type impurities enhancing, oxidation. The need to interpret etching behaviour in a manner consistent with the results obtained by a variety of other microstructural techniques will be emphasized.     

Sliding wear performance of reinforced A413 alloy at elevated temperatures

Dry sliding wear behaviour of Al–Si A413 alloy with and without intermetallics has been studied at ambient and elevated temperatures. It is observed that as the temperature is increased, the wear rate decreased. The reduction in wear rate is mainly attributed to the formation of glazing layers at elevated temperature and is observed in both A413 alloy with and without intermetallics. The wear due to oxidation is predominant during high temperature sliding.     

Synthesis of silicon carbide ceramics from a polysilastyrene at low temperatures

The conventional route for preparation of silicon carbide ceramics is by the use of pressureless sintering, hot pressing, or hot isostatic pressing of silicon carbide starting powders. High sintering temperatures (2073–2473 K) and the addition of sintering additives are normally used to enhance densification. These sintering additives, however, form second phases at grain boundaries which impair the mechanical properties of the material, particularly at high temperatures. It is therefore desirable that new processing routes are developed that overcome these difficulties. A proposed route is to use a polymeric pressure which can provide a Silicon carbide matrix as binding agent for silicon carbide powders, thus making the requirement for high temperatures and sintering additives unnecessary. This paper reports observations of the direct transformation of a polymeric precursor into amorphous Si–C, and crystalline SiC at low temperatures, and the use of this precursor as a binder for the production of SiC powder/ex-precursor SiC composites.     

Friction and wear behavior of laser composite surfaced aluminium with silicon carbide

The present study concerns the wear behavior of laser composite surfaced Al with SiC and Al + SiC particulates. A thin layer of SiC and Al + SiC (at a ratio of 1:1 and dispersed in alcohol) were pre-deposited (thickness of 100 μm) on an Al substrate and laser irradiated using a high power continuous wave (CW) CO₂ laser. Irradiation leads to melting of the Al substrate with a part of the pre-deposited SiC layer, intermixing and followed by rapid solidification to form the composite layer on the surface. Following laser irradiation, a detailed characterization of the composite layer was undertaken in terms of microstructure, composition and phases. Mechanical properties like microhardness and wear resistance were evaluated in detail. The microstructure of the composite layer consists of a dispersion of partially melted SiC particles in grain refined Al matrix. Part of the SiC particles are dissociated into silicon and carbon leading to formation of the Al₄C₃ phase and free Si redistributed in the Al matrix. The volume fraction of SiC is maximum at the surface and decreases with depth. The microhardness of the surface improves by two to three times as compared to that of the as-received Al. A significant improvement in wear resistance in the composite surfaced Al is observed as compared to the as-received Al. The mechanism of wear for as-received vis-à-vis laser composite surfaced Al has been proposed.     

The wear of martensitic stainless steel against tungsten carbide at temperatures up to 500 °C

The bearings in air motors of modern jet aircraft engines must operate dry in hostile conditions at temperatures up to 500 °C. One of the few metallurgical combinations which can function efficiently under these conditions is martensitic stainless steel on tungsten carbide. The work described was initiated to isolate the wear mechanism of such steels in contact with tungsten carbide at elevated temperatures. Experiments were carried out for pure sliding conditions (pin-on-disk experiments) and for rolling-sliding angular-contact bearings such as those used in practice. Wear rates were measured for both configurations for a series of loads, speeds and surface temperatures and extensive X-ray diffraction analyses were carried out on wear debris and on worn surfaces. Three distinct mechanisms of wear were established and found to be present in both configurations. These involve oxidation and abrasive wear at lower temperatures but become heavily dependent on material transfer as temperature increases. It is proposed from the results that the pin-on-disk experiments may be useful as a screening test for the selection of materials without the considerable cost of producing one-off angular-contact components. More evidence from other materials is, however, necessary to establish the validity of the test.     

Study on tool wear characteristics in diamond turning of reaction-bonded silicon carbide

Tool wear is one of the most critical problems in machining hard, brittle materials. In the present work, diamond turning experiments were performed on reaction-bonded silicon carbide, and the tool wear characteristics were investigated. A special kind of wear pattern, namely periodical groove wear, was identified on the flank face of the tool, where the periodicity of the microgrooves was the same as the tool feed. Geometrical analysis showed that the periodical groove wear was caused by the tool feed marks on the machined surface. Laser micro-Raman spectroscopy indicated that the high-pressure abrasive wear at the tool?Cworkpiece interface dominates the wear behavior, rather than the diamond?Cgraphite transformation. By swinging the tool around its curvature center during the cutting process, the periodical groove wear pattern was suppressed, and the tool wear was reduced significantly.     

Sliding wear behaviour of an electrochemically modified austenitic high-nitrogen steel surface

Wear debris from artificial metallic implant joints is known to provoke detrimental foreign-body reactions in the surrounding human tissue. Although commonly used biotolerant metals generate only a little amount of particles, wear is still a major cause for concern. It is the aim of this work to evaluate the sliding wear resistance of a topologically modified high-nitrogen austenitic stainless steel. A disc-on-pin test in self-mating contact is performed in distilled water. Submicron particles are trapped by the structured topography and form together with the plastically deformed metal a hybrid surface which has the potential to significantly improve the tribological behaviour of the tested high-nitrogen steel.     

超声辅助磨削碳化硅陶瓷的工具磨损试验研究

为探究超声辅助磨削过程中不同工具的磨损特征,采用电镀和钎焊两种金刚石磨头对碳化硅陶瓷进行了超声辅助磨削和普通磨削对比试验,研究了超声振动作用、工具类型对磨粒磨损形式及其变化过程的影响,在此基础上分析了磨粒磨损形式对工件表面质量的影响.试验结果表明:对于电镀磨头,普通磨削和超声辅助磨削过程中的磨粒磨损形式均以磨耗磨损和宏观破碎为主,超声振动作用可有效改善加工表面质量;而对于钎焊磨头,普通磨削的磨粒磨损形式主要是磨耗磨损和宏观破碎,超声辅助磨削的磨粒磨损形式主要是磨耗磨损和微破碎,初始阶段超声振动作用可改善表面质量,但随着磨削行程的增加,微破碎形式的占比增高,超声辅助磨削时的工件表面粗糙度值高于普通磨削时的工件表面粗糙度值.     

复合湿法腐蚀工艺制备硅基三维曲面

提出了将各向异性湿法腐蚀与各向同性湿法腐蚀相结合的复合工艺,通过控制刻蚀工艺参数进行体硅加工,成功刻蚀了硅基材料三维曲面回转体结构.在各向同性腐蚀过程中,由各向异性刻蚀得到的多面体结构的表面垂直腐蚀速率与刻蚀液浓度呈指数关系,而搅拌使得多面体结构表面峰值与谷底的刻蚀液存在流速差,基于此原理可得到光滑的三维曲面.刻蚀过程中,通过各向异性湿法腐蚀控制结构深度,通过各向同性湿法腐蚀"抛光"结构曲面.最后,采用实验优化湿法腐蚀过程的工艺参数,基于直径为600～1 000 μm的圆形掩模板,在硅材料表面制备得到了高度为100～200μm的三维曲面回转结构.提出的工艺简单、有效且便于操作,有望用于制作不同曲面形状的三维硅结构及聚合物光学器件模具.     

Friction and wear of PTFE composites at cryogenic temperatures

This paper presents investigations on the tribological behaviour of PTFE composites against steel at cryogenic temperatures. The results showed that the friction coefficient decreases with temperature down to 77 K, but did not follow a linear evolution further down to extreme low temperatures. It can be stated that the cryogenic environment has a significant influence on the tribological performance of the polymer composites. The effect of low temperatures was more clearly detected at low sliding speed, where friction heat is reduced. A change in wear mechanism from adhesive to abrasive was observed in this case. SEM and AFM analyses showed that the PTFE matrix composites investigated under these experimental conditions have transferred material onto the disc down to very low temperatures. Chemical analyses indicate the presence of iron fluorides.     

Tribological characteristics of silicon nitride at elevated temperatures

A sliding friction-and-wear test for silicon nitride (Si₃N₄) was conducted using a ball-on-disk specimen configuration. The material used in this study was HIPed silicon nitride. The tests were carried out from room temperature to 1000°C using self-mated silicon nitride couples in laboratory air. The worn surfaces were observed by SEM and the debris particles from the worn surfaces were analyzed for oxidation by XPS. The normal load was found to have a more significant influence on the friction coefficient of the silicon nitride than an elevated temperature. The specific wear rate was found to decrease along with the sliding distance. The specific wear rate at 29.4 N and 1000°C was 292 times larger than that at room temperature. The main wear mechanism from room temperature to 750°C was caused by brittle fracture, whereas from 750 to 1000°C the wear mechanism was mainly influenced by the oxidation of silicon nitride due to the increased temperature. The oxidation of silicon nitride at a high temperature was a significant factor in the wear increase.     

Effect of surface etching on the lubricated sliding wear of an eutectic aluminium–silicon alloy

The paper presents a study of the formation of wear grooves on near-eutectic aluminium–silicon alloy flats, by sliding a steel ball. The formation of the grooves are tracked on etched and unetched flats as functions of normal load and sliding distance. The groove is initially formed by plastic flow, and then expanded by micro-abrasion as the ball continues to slide on the groove. Etching causes surface hardening of the alloy, but, more importantly creates a surface topology that reduces the peak contact pressure, which discourages further plastic flow in the subsurface. This effect is rationalised using an existing contact mechanical model of indentation of rough surfaces.     

Sliding wear of polymeric composites

The wear mechanisms of two different polymeric composites sliding against metal were investigated experimentally. The sliding distance, normal force and sliding speed were the test variables.Microscope observations showed that the mode of wear of the two materials is similar. Subsurface deformation, crack nucleation at the matrixharder particle or matrix-glass fibre interface, crack propagation parallel to the surface at a depth corresponding to the friction coefficient and crack shearing to the surface were found in both materials. The wear particles often have the shape of thin sheets. Comparison of the metal subsurfaces and wear particles with those of the polymers indicates that the mechanism of wear of semicrystalline composites is similar to delamination wear. Since film transfer greatly influences friction and wear it should be considered together with crack growth in fatigue in the prediction of wear of polymeric composites.     

Sliding wear of quasicrystalline coatings

The mass loss during sliding wear of several quasicrystalline (QC) coatings has been measured. QC coatings of the Al–Cu–Fe, Al–Cu–Fe–Cr and Al–Pd–Mn systems have been investigated and compared with a hardened tool steel and a WC–6% Co hardmetal. The wear rates of the coatings are in general comparable to conventional metallic materials. There is some variation in sliding wear behaviour of different QC coatings. This revised version was published online in August 2006 with corrections to the Cover Date.     

Origins and development of oxidational wear at low ambient temperatures

The origins and the development of the oxidational theory of mild wear under conditions where the ambient temperatures are sufficiently low that no significant oxidation can occur outside the instantaneous real areas of contact between two sliding surfaces are reviewed in this paper. Emphasis is placed on the importance of heat flow analysis for calculating surface temperatures and the division of heat at the sliding interface, especially in so far as it is used for checking the surface model used for explaining the wear rates obtained in some pin-on-disc experiments with low alloy steels. It is shown that it is possible to deduce values for the oxidation constants during wear that are different from those obtained from static oxidation tests and which are relevant to a range of low alloy steels. The implications of some of the computed values of the number N of asperities beneath the pin at any given time, the temperature T_c within the real areas of contact and the critical oxide film thickness ξ are discussed.     

The wear performance of yttrium-modified Stellite 712 at elevated temperatures

Cobalt-based alloys are often used for bearing applications, especially at elevated temperatures. One of the newly developed 700 series cobalt-based alloys, Stellite 712, has been demonstrated to possess high resistance to wear and corrosion in aggressive environments. Continuous efforts have been made to further improve this alloy for enhanced resistance to high-temperature wear involving oxidation. Recent studies showed that the improvement of the oxide scale on Co-base alloys by alloying with yttrium was an effective way to diminish wear of the alloys at elevated temperatures.In this work, sliding wear performances of yttrium-free and yttrium-containing Stellite 712 samples at elevated temperatures were evaluated. The mechanism responsible for changes in its wear performance was investigated by studying the effects of alloying yttrium on microstructure and mechanical properties of the bulk alloy and its oxide scale, employing various experimental methods including micro- and nano-mechanical probing, XRD, SEM-EDS, AFM and high-temperature pin-on-disc wear testing. The research demonstrated that alloying a small amount of yttrium (e.g. less than 1%Y) rendered the oxide scale on Stellite 712 stronger with higher adherence to the substrate, which was largely beneficial to the wear performance of the alloy at elevated temperatures. Mechanisms involved are discussed in this article.     

Sliding wear behavior of Fe-based bulk metallic glass at high temperature

In the past decade Fe-based bulk metallic glasses (BMGs) have attracted increasing attention due to their beneficial properties, including high glass forming ability (GFA), high strength and hardness and high fracture toughness in both fundamental science and engineering application. Most research using these materials has been conducted at room temperature environment, and research that assesses their behavior especially at high temperature has been scarce. We present the results of high temperature effect on the friction and wear behavior of Fe-based bulk metallic glass (BMG), and we tested that this material may satisfy wear and oxidation resistance at high temperature as well as to explore the high temperature wear mechanism of the Fe-based BMG. The dry sliding tribological behaviors of Febased BMG against Si₃N₄ ceramic were conducted with a pin-on-disc friction and wear tribometer. The morphology of the worn surfaces of Fe-based BMG was examined by scanning electron microscopy (SEM) and the chemical composition characterized with energy dispersive spectroscopy (EDS) to observe the wear characteristics and investigate the wear mechanisms. The overall average friction coefficient value generally decreased with increasing temperature, and the glass transition and the formation of protective oxide film played an important role in the tribological behavior of BMG. The wear resistance of Fe-based BMG was not only from their hardness but also from the formation protective oxide layer. Analysis of the worn surface revealed abrasion, plastic deformation and oxidation during sliding test.     

Grooving wear of single-crystal tungsten carbide

The anisotropic nature of tungsten carbide (WC) single crystals has been evaluated in single-tip scratch testing and in multiple-tip abrasion. The single-tip grooves were made with a Vickers diamond indenter and the abrasion tests were performed with diamond and silica grits. All tests were performed on both the prism and basal planes of the WC crystals. A polycrystalline binderless carbide (Bl) was also evaluated. Optical surface profilometry was used to estimate the amounts of displaced, removed and ridge-formatted material in the scratch tests and the wear volumes in the abrasion tests. The scratches and wear scars were studied with scanning electron-, atomic force- and light optical microscopy (SEM, AFM, LOM). In situ studies of the scratch process were also performed. Wear debris were analysed with transmission electron microscopy (TEM). The results show that there are differences in both the amount of wear and the wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms change from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). It is also shown that WC tends to wear by a formation of angular rod-shaped wear debris with the slip planes as the preferred surface planes.     

Mechanisms of sliding wear of metals and alloys at elevated temperatures

Sliding wear at elevated temperature is an important material removal mechanism in large number of engineering applications such as metal forming operation, gas turbine engines, etc. The material loss during sliding at elevated temperature is governed by the antagonistic effect of wear process and oxidation. The objective of the present work is to give an outline of the current status and future trends of wear at elevated temperature of selected metallic materials. Starting with a brief overview of present level of understanding of the elevated temperature wear of various metals and alloys, the initial parts of the paper describes various important development in recent years on elevated temperature sliding wear. The salient features which have helped us to gain in depth scientific knowledge of elevated temperature wear are discussed in the light of recent developments. The overview is further substantiated by detailed study and observation in recent times in particular, the work done at the Vienna University of Technology (Institute of Microtechnique and Precision Engineering) and at the Austrian Center of Competence for Tribology. Specific examples from the recent literatures are described to exemplify the mechanisms of formation of various types of layers during high-temperature wear. Some thoughts on the future directions for research are also outlined.     

Nanoindentation of aluminum (100) at various temperatures

High temperatures generally affect materials in some form. In this regard, the capability to perform nanoscale measurements at elevated temperatures opens up new possibilities for investigating the temperature dependence of materials’ mechanical properties. Particularly, the responses of aluminum’s different mechanical properties to indentation at various temperatures have been studied experimentally. In this paper, aluminum response to different room temperatures was examined. The behaviors of a single crystal aluminum during loading and unloading were observed. Nanoindentation experiments on a single crystal aluminum (100) sample at temperatures of 265 K and 388 K were performed with different loading conditions. At the start of the first burst of the dislocation glide, which was indicated by a sudden increase in displacement with no increase in loading, evidence of plastic properties and softening effects on aluminum was identified. The ductile to brittle transition was observed at temperatures below 273 K. Generally, there was a significant increase in the penetration depth and a decrease in hardness, elastic modulus, and elastic recovery as the testing temperature increased.