Said Jahanmir

Wear transition diagram for silicon nitride

Abstract Utilization of silicon nitride ceramics in applications involving contact between two sliding surfaces requires information on the effect of contact conditions and materials microstructure on tribological performance. In the present study, unlubricated wear tests were conducted on a hot isostatically-pressed silicon nitride under various test conditions in self-mated sliding tests in air. Following the tests, scanning electron microscopy (SEM) was used to elucidate the wear mechanisms and particularly to delineate the effects of load and temperature on wear. The results of the tests and observations were used to construct a wear transition diagram, with load and temperature as the two axes. This diagram is divided into five regions plus one transition zone. The controlling mechanism and tribological data, i.e. friction coefficient and wear coefficient, in each region are unique. At low loads and relatively low temperatures, the tribological behavior is controlled by tribochemical reactions between silicon nitride surface and water vapor in the environment. In the temperature range 400–700°C at low loads, selective oxidation of WC inclusions controls the wear behavior. Formation of crystalline precipitates from the amorphous magnesium silicate grain boundary phase controls the wear process from 700 to 900°C at low loads. At higher temperatures, oxidation of silicon nitride dominates the wear process. A transition to severe wear by micro-fracture is observed as the contact load is increased above a particular value. Detailed understanding of the fundamental mechanisms can provide guidelines for microstructural modifications to avoid severe wear under operating conditions.

Microfracture and material removal in scratching of alumina

A bonded-interface sectioning technique is used to examine subsurface damage modes and to identify mechanisms of material removal in repeated single-point scratching of alumina as a function of grain size, load, and number of passes. In the fine grain alumina, the lateral and median crack system is observed, together with intergranular microcracks and intragrain twin/slip bands distributed within the plastic zone. The distributed form of damage, namely twin/slip bands and intergranular microcracks, are also observed in the coarse grain alumina; but no evidence is found for well-defined median and lateral cracks in this material. The mechanism of material removal in alumina is identified as grain dislodgement resulting from grain boundary microcracking, irrespective of the grain size. Extension of lateral cracks is found to contribute to the material removal process only in the fine grain alumina scratched under a large load and after several passes. A model for the microfracture-controlled material removal process is proposed that relates the volume of material removed to the applied load and material properties including grain size, elastic modulus, hardness, and short-crack toughness. Removal rate is shown to be proportional to grain sizeI1/2 and to loadP2. The model and the experimental results obtained in scratching are used to describe the action of an individual abrasive grit in grinding and other abrasive machining processes.

Enamel Subsurface Damage Due to Tooth Preparation with Diamonds

In clinical tooth preparation with diamond burs, sharp diamond particles indent and scratch the enamel, causing material removal. Such operations may produce subsurface damage in enamel. However, little information is available on the mechanisms and the extent of subsurface damage in enamel produced during clinical tooth preparation. The aim of this study, therefore, was to investigate the mechanisms of subsurface damage produced in enamel during tooth preparation by means of diamond burs, and to examine the dependence of such damage on enamel rod orientation, diamond particle size, and removal rate. Subsurface damage was evaluated by a bonded-interface technique. Tooth preparation was carried out on two enamel rod orientations, with four clinical diamond burs (coarse, medium, fine, and superfine) used in a dental handpiece. The results of this study showed that subsurface damage in enamel took the form of median-type cracks and distributed microcracks, extending preferentially along the boundaries between the enamel rods. Microcracks within individual enamel rods were also observed. The median-type cracks were significantly longer in the direction parallel to the enamel rods than perpendicular to the rods. Preparation with the coarse diamond bur produced cracks as deep as 84 ± 30 μm in enamel. Finishing with fine diamond burs was effective in crack removal. The crack lengths in enamel were not significantly different when the removal rate was varied. Based on these results, it is concluded that subsurface damage in enamel induced by tooth preparation takes the form of median-type cracks as well as inter- and intra-rod microcracks, and that the lengths of these cracks are sensitive to diamond particle size and enamel rod orientation, but insensitive to removal rate.

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,...

In-Vitro Contact Wear of Dental Composites

OBJECTIVE The aim of this study is to determine the in vitro two-body contact wear mechanisms of three medium filled composites and compare these with a highly filled composite previously investigated. MATERIALS AND METHODS Three commercial dental composites with filler mass fraction loading of 75-76% were evaluated. Two of the composites contained Ba-B-Al-silicate glass fillers and fumed silica with different particle sizes and distributions. One of these composites contained a fairly uniform distribution of filler particles ranging in size from 1 to 5 microm, whereas the particle size distribution in the second composite was bimodal consisting of small (less than 1 microm) and large (about 10 microm) particles. The third composite contained Ba-Al-silicate glass and silica with a filler particle size of approximately 1 microm. The composite disks were tested for wear against harder alumina counterfaces. Wear tests were conducted in distilled water using a pin-on-disk tribometer under conditions that represented typical oral conditions (sliding speed of 2.5 mm/s and contact loads ranging from 1 to 20 N). The wear tracks were analyzed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy to elucidate the wear mechanisms. The chemical composition of the water solution collected after the tests was determined using an inductively coupled plasma-mass spectrometer (ICP-MS) to detect possible chemical changes, e.g. dissolution of trace elements due to submersion or wear. The wear results were compared with those reported in an earlier study on a highly filled composite containing predominately alumino-silicate glass fillers and alumina at a filler loading of 92%. RESULTS The differences in two-body wear rates between the three medium filled composites were not statistically significant (p<0.05) indicating that the variations in filler particle size and slight differences in chemical composition of the glass fillers do not affect the in vitro wear rates of these composites. Wear rates of these medium filled composites, however, were significantly lower than the highly filled composite (p<0.05). SEM, FTIR and ICP-MS analyses suggested that wear in the medium filled composites occurs by a complex set of processes involving tribochemical reactions between filler particles and water, formation of surface films containing a mixture of filler fragments and reaction products, and film delamination, as well as dissolution of the reaction products. SIGNIFICANCE This study reveals that subtle changes in the filler particle size and small differences in filler composition do not significantly affect the two-body wear behavior of medium filled composites. However, the chemistry of filler particles plays an important role in altering the wear performance of composites when significant changes are made in the chemical composition of the fillers and when the filler loading is increased.

Wear transition diagram for silicon carbide

Abstract To obtain information on the tribological behaviour of silicon carbide at elevated temperatures, unlubricated ball-on-flat wear tests were conducted on sintered silicon carbide in self-mated sliding in air. The contact load was varied from 3.2 to 98.0 N, and a temperature range of 23°C to 1000°C was used. Scanning electron microscopy, Fourier transform infrared spectroscopy and energy-dispersive spectroscopy were used to elucidate the wear mechanisms. The results of the tests and observations were employed to construct a wear transition diagram, which provides a summary of tribological information including friction coefficient, wear coefficient and wear mechanisms as a function of temperature and load. The wear transition diagram for the sintered silicon carbide studied is divided into four regions plus one transition zone. At room temperature, under high loads and high environmental humidity, the tribological behaviour is controlled by tribochemical reactions between the silicon carbide surface and water vapour in the environment. Under low loads and at temperatures below 250°C, wear occurs by ploughing and polishing. At temperatures about 250°C and under low loads, tribooxidation and formation of cylindrical wear particles control the tribological behaviour. Wear occurs by microfracture when the load is increased above a critical value; and both the friction coefficient and the wear coefficient increase.

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.

Effect of contact pressure and load on wear of alumina

Wear experiments were conducted in air using self-mated alumina in a pin-on-disk configuration at different loads using balls of different diameters as pins to maintain a constant value of initial Hertzian pressure. The selected initial load was kept constant for the rest of the sliding. Wear volume on the ball and the apparent contact pressures were calculated from the measured wear scar diameters on the balls at regular sliding intervals. The results show reduction in wear coefficient with sliding distance, as the contact pressure reduces (due to increment in the contact area). However, a good correlation was observed between the normalized wear rate and the apparent contact pressure, irrespective of the applied load, sliding distance, ball diameter and apparent contact area. This observation is explained based on the assumption that besides the asperities that deform plastically a large portion of the asperities deforms elastically and carries the applied load. It is argued that for a mixed elastic/plastic contact the number of plastically loaded asperities, and therefore, the wear rate, should depend on the apparent contact pressure.