Papers by Author: Jia Wen He

Abstract: Microdrills were deposited by Cr-N based and TiN coatings using a magnetron sputter technique. A flexible printed circuit board was used to evaluate the drilling lives. SEM observation was employed to reveal the wear and local welding. A special pin-on-disc tester was designed to assess the interaction between coatings and against materials. Drilling test indicated that the number of drilling holes increased significantly when the minidrill was coated Cr-N based film than TiN. Failure model of microdrill was studied for cutting this copper rich printed circuit board. Results demonstrated that welding produced between TiN coating and rich copper counter material, but welding did not occur between Cr-N based film and copper material.

Abstract: Repeated impact test is an appropriate way to evaluate coating properties of metal cutting tools since the cyclic impact loading applied in the test simulates the interrupted cutting process. For hard CVD coatings with multi-layered structure, the alumina is usually of the lowest toughness and adhesion, and relatively easy to fail in the metal cutting. In the present paper, multilayered coatings consisting medium temperature TiCN, Al2O3 and TiN were deposited by chemical vapor deposition (CVD) on the tungsten carbide/cobalt substrate and evaluated using scratch adhesion test and repeated impacted test. The phase structure of alumina layers are α or )κ as usually applied in cutting tools, and the grain size for the α phase one is varied from one micron to four microns. The scratch and indentation test results show that the alumina layer of κ phase is easy to remove under the quasi-static loading, implying brittleness and low adhesion. The repeated impact test results reveal lower impact resistance for the κ phase alumina, and only adhesive failure can be found, which differs to that of the α phase one. The coarse grained α-Al2O3 layer shows similar adhesion compared to the fine grained α- Al2O3 in scratch test. The failure mode of the coating with coarse grained α-Al2O3 layer in the repeated impact test is primarily impact wear. No cohesive failure can be found at all load levels, and this leads to higher impact life than that of the fine grain one. It was shown that the relatively low adhesion of bottom TiCN layer may not affect the impact resistance.

Abstract: CrxN coatings are deposited on the substrates with low temperature using a Teer UDP 550 Close Field Unbalanced Magnetron Sputter Ion Plating system (CFUBMSIP). The effect of applied power on Cr targets related to sample temperature is studied. When the applied power of Cr targets is lower than 1.1KW, the depositing temperature could be controlled at about 180°C. Scratch test shows no coating detachment at the end of scratch track up to 85N load. All results of Rockwell-C indentation indicate that CrxN coating holds good bonding strength. Pin-on-disc tests are performed to evaluate wear resistance of the coatings and no any wear loss can be detected after 6000s and 1800s of running. XRD profile demonstrates that the coating deposited in this study is CrxN.

Abstract: The stress field of an interfacial crack in non-homogeneous materials is computed using the semi-analytical method of arbitrary lines (MAL). Then, the eigen-functions of stress, strain and displacement, i.e. the angular distribution functions near a crack tip, are analyzed based on our wedge-shape non-homogeneous model. Finally, the growing direction of the interfacial crack is determined according to the relevant maximum normal stress criterion accurately. Therefore, the effective approach is provided for solving the complicated crack growing directions of an interfacial crack in non-homogeneous materials.

Abstract: The effect of different continuous material parameter functions in the non-homogeneous region on the eigen-functions of crack tip fields is studied by means of the semi-analytical numerical method based on the wedge shaped non-homogeneous sector model. The cases of the different opening angles of the non-homogeneous region, the different dissimilar material ratios, the different configurations, etc. are analyzed systematically. The result shows that the effect becomes smaller as the opening angle of the wedge shaped non-homogeneous sector becomes smaller. The valuable conclusion is drawn that the differences among the eigen-functions corresponding to the cases can be neglected as long as the opening angle is small enough for the engineering application, which facilitates the engineering application of the non-homogeneous region model.