Research On Precision Milling Of Zirconia Ceramics By Micro Diamond End Mills

Advanced engineering ceramics have become the indispensable structural materials that with great application potential in the field of defense, aerospace, aviation and other modern high-tech industries. Due to high hardness and poor toughness, engineering ceramics are generally difficult to cut, and easier to fracture during machining process. At present, grinding is still the main machining method for engineering ceramics. Electrical discharge machining(EDM), ultrasonic/laser assisted machining etc. processing methods are also used for engineering ceramics machining, which makes up the limitations of grinding to some extent. Ductile mode precision milling of engineering ceramics is an expand of milling process in the area of ductile machining of hard brittle material, which not only could meet the demand of machining real 3D features, but also can achieve high quality surface as well as precision grinding. Based on the principle of ductile mode machining, fundamental research works on precision milling of ZrO2 ceramics with ultra-hard end mills are carried out in this paper. The main contents are as follows:(1) Based on the ductile mode removal mechanism of brittle materials, the difference between precision milling and traditional milling from the scale features was analyzed. The selection principle of precision milling parameters that to limit the maximum un-cut chip thickness(hmax) was established. Compared with the variation of milling depth(ap), the variation of feed per tooth(fz) gives greater effect on the changes of maximum un-cut chip thickness. A method to measuring the critical depth of cut(dc) of brittle materials by evaluating the changes of cutting force during the incremental cutting depth cutting process was proposed. And the critical depth of cut(dc) of ZrO2 ceramics was successfully measured by the tests then.(2) Experimental researches on precision milling of ZrO2 ceramics with CVD diamond coated end mills and self-development PCD micro end mills were carried out. Ductile mode micro milling of ZrO2 ceramics was realized by controlling the maximum un-cut chip thickness. The machined surface show mirror-like quality, and the achieved minimum surface roughness can be down to Ra 0.016μm. The effects of milling parameters, milling length etc. on machined surface quality were also investigated. It was found that the feed per tooth influenced the surface roughness most.(3) The micro morphological features of ductile machined surface, including material plastic side-flow, periodic feed marks etc. were observed by scanning electron microscopy(SEM). The surface formation mechanism under the condition of considering tool corner radius during precision milling process was analyzed. Surface defects such as micro pits, cracks, and edge collapse during precision milling of ZrO2 ceramics were also observed. It was revealed that the brittle fracture defects were mainly formed by the intergranular cracking, while the smooth surface areas on the machined surface were mainly formed by transgranular cracking.(4) The tool wear performance of diamond coated end mills and PCD end mills in precision milling of ZrO2 ceramics were investigated. For diamond coated end mills, the tool wear process included three stages: initial chipping of diamond coating, large chipping of diamond coating and wear of substrate WC. The premature delamination of diamond coating was the main fact which affect the performance of diamond coated end mills. It was found the fractured diamond coating edge also could remove materials as cutting edge. For PCD end mills, the tool wear mechanism was abrasive wear and adhesion of ZrO2 ceramics. Tool wear was affected by milling parameters, PCD grain size etc. At the same time, tool wear had influence on cutting force, machined surface roughness, and the accurate of the machined groove’s width.