Model Of Surface Roughness In Fixed Abrasive Lapping Of Brittle & Hard Material

Brittle & hard material is the most widely used functional material. It’s very easy to break during processing for the characteristics of high hardness and brittleness. The formation of cracks and pits on the surface badly affects the surface quality of workpiece and device performance. The abrasive like diamonds, cerium oxide, etc. are bonded in the lapping pad substrate instead of scattering in the slurry. During the lapping process, the workpiece contacts with the pad under applied pressure and the material was removed in brittle-plastic way through the exposed abrasive in bond layer, which significantly reduces surface roughness of the workpiece. It has remarkable superiority in ultra-precision machining of hard and brittle material.Surface roughness is an important indicator to measure the surface quality of the parts and also the common parameter used to describe the processing quality in ultra-precision machining. The model of surface roughness in lapping of brittle & hard material was established to improve processing efficiency and save production cost. In this thesis, fixed abrasive lapping process is simplified, statistics and mechanics principle were used to establish a surface roughness model of fixed abrasive lapping of brittle & hard material based on surface microstructure of the lapping pad. The model was verified by fixed abrasive lapping of K9 glass. The specific research content and results are as follows:(1) The lapping process was simplified and the distribution of abrasive particles was presumed to research effective grain number in lapping process. On the base of grain-workpiece contact model, force balance principle was used to study the relationship between abrasive cut depth and lapping parameters, as well as characteristic parameters of the pad.(2) On the base of brittle & hard materials removal mechanism, the surface forming of lapping was assumed reasonably. The contact between surface roughness and particle cut depth was obtained by using probability theory and mathematical statistics according to the definition of arithmetical mean deviation of the profile(Ra). The mathematical model of the surface roughness was established by combining the contact between the particle cutting depth and lapping parameters as well as the pad characteristic. The model shows that surface roughness value of the workpiece is proportional to particle size and one-third power of applied pressure, inversely proportional to two-ninth power of the pad concentration, and also related to the workpiece hardness and pad bulge density.(3) The model was verified by lapping K9 glass. Controlling the changes of process parameters(abrasive grain size, lapping pressure and pad concentration), K9 surface roughness value under different process parameters were analyzed and the influence rules about one of the process parameters on the surface roughness were obtained. The results show that the changing trend of theoretical surface roughness values over abrasive particle sizes and pad concentrations as well as applied pressure matches the experimental trend, which proves the reliability of the model.(4) For the difference between experimental values and theoretical ones, considering the effect of abrasive particle trajectory and pad elastic, the groove overlapping rate and the pad elasticity coefficient were introduced to modify the theoretical model. The modified surface roughness value decreases with the increase of groove overlapping rate, and increases as elastic modulus of the pad rises. Under selected experimental parameters, the error between the calculated values and experimental values could be controlled below 5%. The calculation precision was improved by about 35% than that before.