Steven F. Wayne

Characteristics of metallic subsurface zones in sliding and impact wear

Abstract Various metallic pairs were tested under conditions of unlubricated solid contact. Experiments were conducted for repetitive impulsive and continuous sliding contact. Wide ranges of materials and conditions (nominal contact stress and relative transverse sliding velocity) and a variety of loading modes (pure normal impact at various frequencies, compound impact at various sliding velocities, and pure sliding under various stress levels) were explored. Particular attention was focused on the establishment of subsurface material zones developed in the tests, in situ . These zones exhibit dependences on velocity, stress, material, test duration and loading mode. The experimental findings, based on several analysis techniques, serve to characterize subsurface zone composition and morphology. Both surface and subsurface features were examined by optical and electron microscopy and analyzed by energy-dispersive X-ray techniques to allow interpretations concerning the role of external parameters, material transport and debris formation, as well as insight into operative mechanisms which act on specific materials under prescribed conditions to cause wear.

Formation of Subsurface Zones in Impact Wear

Recent studies have established that subsurface material structures are altered in processes giving rise to wear. This is the case both for sliding contact and repetitive compound impact loading. It is not surprising that the thermal and mechanical processing due to solid contact and relative motion lead to localized material property change. Yet subsurface material characteristics (such as degree of plastic deformation, void and crack content, layer formation, grain refinement, etc.) may be of considerable significance in terms of fundamental wear processes. Accordingly, a series of tests has been undertaken to determine the onset of characteristic subsurface zone formation arising in repetitive compound impact wear. The repetitive load cycling is convenient for such investigations (incubation studies) since a contact event is discretized a priori. The test series and subsequent analyses seek primarily to determine the dynamics of formation of subsurface material zones, and also to characterize these zon...

The role of microstructure in the wear of selected steels

Abstract This investigation is a continuation of impact wear studies which focus on the nature of subsurface microstructure. Both AISI 1045 and 2.25Cr-1Mo steels were selected for their capacity to form various phase morphologies at given compositional states. Heat treatment was then performed to produce the desired two-phase (duplex) structure in both materials. The mating counterface to each test material was a 17-4 PH stainless steel in the martensitic condition. Compound impact wear tests were performed at relative transverse sliding velocities of 1 and 10 m s −1 with peak nominal contact stress maintained at 69 MPa for various numbers of repetitive load cycles. The formation and characterization of subsurface zones were studied by scanning electron microscopy and energy-dispersive X-ray analysis. Wear debris was inspected by powder X-ray diffraction. The impact wear resistance of AISI 1045 and 2.25Cr-1Mo steels is dependent on transverse velocity. Variations in velocity lead to “trade offs” between specimen and counterface 17-4 PH stainless steel wear which is evidenced in weight loss data and correlates with microstructural observations (subsurface zone formation) for each two-phase system. Wear debris analysis confirms the presence of mechanochemical material interaction between specimen and counterface with increasing transformation and oxidation at the higher transverse sliding velocity.

The role of specimen stiffness in sliding and impact wear

Abstract The results of pin-on-disc sliding tests and of impact wear tests are presented. Titanium alloy specimens were used for the sliding tests and high strength steel specimens were used for the impact tests: in both types of test stainless steel was the counterface material. The test duration, the nominal contact stress and the effective “stiffness” of the pins were varied; the effective stiffness was varied by changing the unsupported length of the pins while all other experimental conditions were maintained invariant. Experimental data on the wear track depth and roughness, obtained by profilometry, and on the specimen and counterface surfaces, which were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) techniques, are presented: the data include morphological features and material transport observations. Subsurface sections of specimens were analyzed by SEM, EDX techniques and transmission electron microscopy. Characteristic subsurface zones are identified and described with respect to both morphology and composition in the near-surface microstructure. The data clearly indicate that the stiffness is an important factor in experimental work in sliding as well as in impact wear.

Comparative in vitro Sliding-wear Study of Conventional, Microfilled, and Light-cured Composite Restoratives

The sliding-wear behavior of a number of commercially available microfilled and light-cured composite restoratives has been investigated and compared with the wear characteristics of conventional composites. The surface profiles of the composite restoratives, both pre- and post-test, have also been examined in order to study material attrition processes. The results indicate that: (1) microfilled composites are significantly more resistant to sliding wear than are restoratives containing conventionally-sized filler particles; (2) light-cured, microfilled composites exhibit a lower rate of sliding wear than do self-curing, microfilled restoratives; and (3) the wear process leads to an increase in surface roughness for all materials tested, but microfilled materials display lower surface roughness values, both before and after sliding-wear tests, than do restoratives containing conventional-sized filler particles.

Specimen material reversal in pin-on-disc tribotesting

Abstract Results from pin-on-disc wear tests in which specimen and counterface materials were interchanged are presented in this paper. In one set of tests, pins of material A were slid against discs of material B; then, in another set of tests, pins of material B were slid against discs of material A. There are many factors to consider in such experiments, including the fact that the pin experiences a greater sliding distance and presumaly higher near-surface temperatures. However, test results from such specimen material reversals are useful in studying the roles of thermal, mechanical and chemical processes in sliding wear and also in assessing the validity of tribotesting against a “standard” counterface material. The results reported include data on volume change (obtained from weight change measurements), on wear track profiles, on subsurface zone morphology and composition and on debris composition. As expected, these data indicate that significantly different results will be obtained for a given pin-on-disc test for pin material A against disc material B compared with pin B against disc A.

Material transport phenomena in the impact wear of titanium alloys

Abstract Results are presented from compound impact wear studies performed with titanium alloys of different β phase content and morphology. The “material pair” consisted of a 17-4 PH stainless steel counterface and a flatended titanium alloy specimen. Each material pair was exposed to variations in relative transverse sliding velocity and number of repetitive load cycles. Testing was conducted primarily at a single level of nominal peak normal impulsive stress. Both optical and scanning electron microscopy were used to monitor changes in worn surface and subsurface regions. Energy-dispersive X-ray studies of the initial and worn surfaces comprising the material pair clearly indicated the nature of the material transport between the opposing surfaces. Wear debris were studied by optical microscopy and by powder X-ray techniques. Utilizing the reciprocating impact wear testing apparatus, it was determined that material transport appears to be a controlling factor. The type of transport ( i.e. material passing from specimen to counterface or vice versa) was found to vary under differing test conditions. Such findings may contribute to the understanding of wear for systems other than those characterized by repetitive impulsive contact. Material removal is minimal at particular levels of relative transverse sliding velocity, and these levels are not necessarily affected by the magnitude of the nominal level of stress. It appears that the nature and quantity of the constituents (α, β) in the titanium alloys are critical in establishing wear behavior for the material pairs investigated.

Wear of two titanium alloys under repetitive compound impact

Abstract This paper presents results obtained in testing two titanium alloys in compound impact wear. Relative transverse sliding velocity is found to be a significant parameter and distinctive near-surface microstructural features are noted for the two materials. For one alloy preliminary transmission electron microscope (TEM) foil studies indicate the surface layer to be crystalline. TEM replica studies suggest a continuum of deformation in the substrate for both materials.

Wear of copper and copper alloys under repetitive impact sliding

Abstract In this paper we present morphological and compositional findings for surface and sub-surface zones produced in repetitive impact sliding between Cu-0.58at.%Cr and Fe-17Cr-4Ni-4Cu precipitation-hardened steel. The compositional aspect was further studied by using a simplified material pair consisting of pure copper and pure nickel. Scanning electron microscopy suggests the formation of a lamellar surface morphology and energy-dispersive X-ray analysis confirms material transport between the surfaces. Cross sections of CuCr specimens clearly show extensive plastic deformation, yet subsurface voids or cracks are not evident. X-ray analysis of the CuNi worn surfaces provides evidence of mechanochemical mixing in the near-surface zone and indicates that diffusion is most probably a minor contributing mechanism in the formation of compositionally distinct subsurface zones under the test conditions explored.

The crystal structure of the molybdenum cementite Mo12Fe22C10 (?-phase)@@@Die Kristallstruktur von Molybdn-Zementit, Mo12Fe22C10 (?-Phase)

The crystal structure of molybdenum cementite Mo12Fe22C10 (ξ-phase) has been determined by means of a single crystal x-ray diffraction study of crystal fragments. The lattice parameters were found to be:a=10.865 (3),b=7.767 (2),c=6.559 (2) Å and β=120.13 (2)°, space group C2/m;Z=1. From the analysis ofPatterson maps and differenceFourier analysis the atomic parameters were derived, yielding a residual ofR=0.059. The crystal structure contains octahedral and triangular prismatic groups which accommodate the carbon atoms in their voids, as is usually found in interstitial compounds. The octahedral building group consists of four Mo- and two Fe-atoms, the triangular prism is built up by four Fe-and two Mo-atoms. The mode of filling of the metal polyhedra is discussed.ZusammenfassungDie Kristallstruktur von Molybdän-Zementit, “Mo12Fe22C10” (ξ-Phase) wird auf Grund von Einkristall-Beugungsaufnahmen unter Anwendung vonPatterson-and DifferentialFourier-Analysen bestimmt. In der monoklinen Elementarzelle (a=1.870;b=7.67;c=6.563 Å, β=120.1°) Raumgruppe C 2/m befindet sich eine Formeleinheit Mo12Fe22C10 (oderZ=2, Mo6Fe11C5). DerR-Wert von 6% für 1200 Reflexe unterstreicht die Richtigkeit der Struktur, die aus oktaedrischen und trigonal prismatischen Gruppen aufgebaut ist. Die Oktaedergruppe besteht aus 4 Mo- und 2-Fe-Atomen, die trigonal prismatische Gruppe aus 4 Fe- und 2 Mo-Atomen. Die Kohlenstoffatome füllen die Lücken dieser Bauelemente, wie es für typische Einlagerungscarbide (Komplexcarbide) erwartet werden kann.