Lewis K. Ives

Measurement of solid particle velocity in erosive wear

Abstract A method is described for determining the velocity of solid particles in a gas-particulate stream applied to erosion testing of materials. A simple mechanical configuration allows the measurement to be made under a wide range of equipment conditions. The time-of-flight of the particles is determined over a controlled path length between two rotating disks. Examples of measurements on several test apparatus are presented. The importance of nozzle design is discussed. A comparison between particle and gas stream velocity is presented.

Tribological characteristics of synthesized diamond films on silicon carbide

Abstract The purpose of this research was to explore the use of synthesized diamond films as wear-resistant, low friction materials for tribological applications. The friction and wear of diamond-coated SiC sliding against SiC were studied. The diamond films were deposited by means of the hot filament chemical vapor deposition process. A ball-on-three-flat contact geometry was used in the wear experiments. The experimental results showed that the wear rate of SiC disk specimens was reduced by four orders of magnitude when they were coated with a diamond film. Similarly, the friction coefficient was reduced by almost one order of magnitude. Energy-dispersive X-ray analysis of the worn surface of the diamond indicated that the SiC counterface undergoes tribochemical reactions with the air atmosphere producing silicon oxide. Formation of this tribochemical reaction product cannot be responsible for the low coefficient of friction, since the same material is formed in SiC-SiC tests. It is, therefore, hypothesized that the low friction coefficient of diamond may be related to the formation of a thin film of graphite at the real area of contact. Wear of the diamond film is then accounted for by the loss of this graphite layer.

MATERIAL REMOVAL AND DAMAGE FORMATION MECHANISMS IN GRINDING SILICON NITRIDE

Surface grinding was performed on two silicon nitrides with different microstructures. The ground surfaces of both materials were observed with scanning electron microscopy (SEM) to consist of areas of microfracture, smeared areas, and areas covered with fine debris particles. It was determined that microfracture is the primary mechanism for material removal. Subsurface grinding damage was revealed by a bonded-interface technique to take the form of median-type cracks extending from the plastic zones. Distributed intergranular microcracks and intragrain twin/slip bands were observed within the plastic zones. The strengths of transverse-ground specimens were measured in four-point flexure. For the silicon nitride with a fine grain size and a mildly rising toughness-curve, grinding damage resulted in a drastic strength degradation compared to polished specimens. In contrast, the silicon nitride with large and elongated grains and a steeply rising toughness curve showed relatively little strength loss. The relationship between the ceramic microstructure and the damage tolerance in abrasive machining is discussed in light of these results.

EFFECT OF GRINDING ON STRENGTH OF TETRAGONAL ZIRCONIA AND ZIRCONIA-TOUGHENED ALUMINA

Abstract The effect of grinding-induced damage, on the strength of a yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and a zirconia-tough-ened alumina (ZTA) was investigated. The four-point flexure test was used to measure fracture strength as a function of grinding conditions. Flexure bar specimens were prepared by surface grinding transverse to the tensile stress direction in flexure testing. Two series of grinding experiments were performed. In the first series, four wheels with diamond particle sizes ranging from 25 to 180 μ,m were used to grind specimens at a fixed depth of cut. The strength of the Y-TZP decreased slightly with increasing diamond particle size. For ZTA the strength loss was increased as diamond particle size was increased. In the second series, one grinding wheel with a diamond particle size of 180 μm was used to grind specimens at a depth of cut per pass ranging from 2.5 to 100 μm. The Y-TZP showed a slight decrease in strength when the depth of cut was increased. However,...

Transmission and scanning electron microscopy studies of deformation at erosion impact sites

Abstract Scanning and transmission electron microscopy methods have been employed to study topographic features and subsurface damage associated with erosive-particle impact craters in annealed 310 stainless steel surfaces. Angular Al 2 O 3 and spherical glass particles approximately 50 μm in diameter were projected at a velocity of 59 m s −1 to impact the surface at attack angles of 90° and 20°. Under these conditions, material was found to be displaced but not removed from the surface at isolated impact sites. A comparison was made with damage produced at diamond pyramid hardness indentations. Substantial differences were not observed. In general, a high dislocation density zone a few microns wide was found to surround both impact craters and hardness indentations. The width of this zone varied according to the size and shape of the crater and the direction of particle motion. Deformation twinning occurred at some impact sites. The plastic strain associated with impact craters in 310 stainless steel and copper was also determined by a method that is based on an analysis of selected-area electron channelling patterns.

ABRASIVE MACHINING OF GLASS-CERAMICS WITH A DENTAL HANDPIECE

Abstract Dental restorations are commonly prepared from machinable glass-ceramics using modern dental CAD/CAM systems. Unfortunately, little is understood about the influence of machining parameters on material removal rates and any damage which could be introduced into the restoration during the abrasive machining processes employed with these systems. These effects are investigated for three experimental machinable glass-ceramics with varying microstructure and one closely related commercial material. Abrasive machining is performed with dental burs containing coarse and fine diamond particles. The results show that the microstructure of the glass-ceramic, the size of diamond grit in the burs, and the load applied to the burs during machining have significant effects on the machining behavior. By increasing the size of the mica platelets within the glass-ceramics or by increasing the load on the burs, material removal rate increases. However, chipping damage at groove edges increases as either the load is increased or as the size of the mica platelets is decreased. The use of coarse burs does not necessarily result in high material removal rates but increases the extent of chipping damage. Surface roughness is found to be relatively independent of the microstructure or applied load but is strongly dependent upon coarseness of the diamond particles in the burs.

ON THE NATURE OF MACHINING CRACKS IN GROUND CERAMICS: PART I: SRBSN STRENGTHS AND FRACTOGRAPHIC ANALYSIS

ABSTRACT Machining cracks in ground sintered reaction-bonded silicon nitride (SRBSN) rods and bars were analyzed by fractographic techniques. Grinding flaw sizes were as small as 12 µm and as large as 80 µm and correlated strongly with grinding direction and wheel grit size. Some grinding treatments had no deleterious effect on strength since the machining cracks were very small and fracture occurred from the materials inherent flaws. The telltale signs of machining damage may be detected with conventional low power optical microscopy using simple fractographic techniques. The telltale signs are summarized in a new series of schematic drawings which will aid pattern recognition for engineers and fractographers.

ABRASIVE MACHINING OF GLASS-INFILTRATED ALUMINA WITH DIAMOND BURS

The abrasive machining characteristics of a glass-infiltrated alumina used for fabrication of all-ceramic dental crowns were investigated using a high-speed dental handpiece and diamond burs with different grit sizes. The material removal rate, surface roughness, and extent of edge chipping were measured as a function of grit size. The removal rate decreased substantially with decreasing bur grit size from supercoarse (180 μm) to fine (40 μm) and ultrafine (10 μm). The removal rate with the supercoarse burs was approximately twice that achieved with the fine burs and four times the removal rate with the ultrafine burs. Both surface roughness and edge chipping damage were sensitive to diamond grit size. Chipping damage was severe and the surface roughness substantial with the supercoarse burs, while negligible edge chipping and smooth surfaces were obtained with the ultrafine burs. The removal rate also decreased with continued machining for all grit sizes. The observed reduction in removal rate was found to be primarily due to wear of the diamond grit and accumulation of debris on the bur (i.e., bur loading). After prolonged use, a significant loss of diamond grit was observed that led to a substantial loss of cutting efficiency. It is concluded that, with respect to material removal rate and surface integrity, diamond machining is a feasible machining process for glass-infiltrated alumina in the final infiltrated state. However, caution should be exercised in the use of diamond grit larger than 40 μm. Such burs may result in excessively rough surfaces, chipped edges, and strength limiting surface and subsurface microcracks.

Sputtered amorphous carbon nitride films

The recent announcement of the synthesis of C 3 N 4 has increased interest in this unique material. Carbon nitride may have several useful applications as wear and corrosion resistant coatings, electrical insulators, and optical coatings. We have produced amorphous carbon nitride coatings containing up to 40% nitrogen using planar magnetron RF sputtering with and without an ion beam in a nitrogen atmosphere. Both wavelength dispersive x-ray spectrometry (WDX) and x-ray photoelectron spectroscopy (XPS) indicate this composition. Coatings up to 2 μm thick were produced on alumina, silicon, SiO 2 , and glass substrates using a graphite target. Films with transparency greater than 95% in the visible wavelengths and harder than silicon have been produced. The properties of these films are correlated with composition, fabrication, conditions, and subsequent heat treatments. A scanning tunneling microscope (STM) and transmission electron microscopy (TEM) were used to characterize the morphology of the films. XPS studies confirm the stability of a carbon nitrogen phase up to 600 °C. Compositional variations were determined with secondary ion mass spectrometry (SIMS) depth profiling, and the Raman spectra are compared with those of carbon and carbon nitride films prepared by other methods.

Simulation of diamond-ground surfaces

Abstract State-of-the-art models available for predicting the characteristics of diamond-ground surfaces entail digitization of wheel topography which limits their practical utility. This paper presents a geometric simulation of surface generation in diamond grinding, that utilizes wheel topography data obtained from simulated three-dimensional structure of the diamond grinding wheel. Simulation results are validated by surface grinding experiments on a variety of materials that exhibit different material removal mechanisms, with the roughness parameters Ra and Rt, the autocorrelation function and the fractal dimension as the comparison indices. A parametric study on the effect of various grinding parameters on surface finish is also reported, with particular emphasis on identifying and minimizing process-inherent variability.