Basic Research On Mechanical Properties Of Thick Diamond Films Fabricated By Plasma Jet CVD

Since 1990s,CVD diamond film deposition technology has developed rapidly.So far,except some mechanical properties,the properties of CVD polycrystalline diamond films have been comparable to those of natural diamonds,so mechanical properties have always been an important research direction in the field of diamond.Due to the low deposition rate of diamond films,the thickness of free-standing diamond films used in mechanical properties research is usually thin.In view of the great effect of films thickness on mechanical properties and the tremendous market demand for large-size free-standing diamond films,the study of mechanical properties of free-standing diamond thick films is of great significance.The study of mechanical properties in this work mainly includes two aspects:fracture strength and fracture toughness.For the convenience of distinction,free-standing diamond thick films with diameters greater than 80 mm and deposited thicknesses greater than 1 mm are called diamond plate in this paper.Polycrystalline diamond films with various thicknesses were prepared by direct current arc plasma jet CVD(plasma jet CVD)for fracture strength study.The maximum fracture strength of diamond plate with original thickness of 1.47 mm can be 996 MPa,which is the maximum reported fracture strength of the polycrystalline diamond plate of such thickness.The study shows that the main mechanism of high fracture strength is the growth direction change of multiple twins,which results in the optimization of the grain size.Therefore,the small size grains could occupy the space where the large size grain were set or fill the grain gap in competitive growth,and finally causes the formation of grain mosaic form,and this is the major reasons of diamond film fracture strength.Pre-cracks in length range(35%to 60%of sample width)in accordance with international standard for fracture toughness have been successfully fabricated in diamond plate samples with multiple twin morphologies by single-edge pre-cracked beam method.The four-point bending fracture toughness of diamond plate with sharp pre-crack is 11.0 MPa·m1/2,which is higher than that obtained by three-point bending with pre-crack 8.4 MPa·m1/2.The three-point bending fracture toughness is also higher than the result reported previously with thickness 0.25 mm.The fracture toughness of diamond plate is improved because of multiple twins.In addition,the fracture toughness of nucleation surface loading is improved due to the influence of multiple twins,which is next below that of growth surface loading,and the fracture toughness of edge loading is the smallest.Based on the size limitation,the non-standard three-point bending method has been applied to measure the strength of CVD free-standing diamond films in the past two decades.In order to obtain the intrinsic strength of diamond plate accurately and to meet the international standard of fine ceramics,ultra-thick diamond plate with an average thickness of 6.46 mm and a diameter of 120 mm was deposited by plasma jet method,the thickness of which is the maximum of large-size CVD polycrystalline free-standing diamond film in current literature.After polishing long face and long edge to appropriate accuracy(nucleation surface roughness Ra?15 nm,long edge with 45-degree chamfer,long face parallelism and perpendicularity ?15 ?m),diamond standard sample(ISO standard sample)which conforms to the international standard ISO 14704:2000 for three-point bending fracture strength of fine ceramics was prepared for the first time,and its size is 36 mm × 4 mm × 3 mm.The three-point bending fracture strength of ISO standard diamond sample is 581 MPa for the first time,which is much larger than that of ISO standard size multispectral ZnS strength 81 MPa and ?-Al2O3 plate strength 120 MPa.The test of fracture strength of ISO standard sample can provide data support for mechanical application of diamond plate.The improvement of fracture strength and toughness of diamond plate by low-pressure/high-temperature treatment was studied.After treatment,the maximum fracture strength of diamond film with polishing thickness of 1.51 mm increased from 700 MPa to 1030 MPa.Microstructure analysis shows that the increase of stress caused by graphitization during heat treatment is the main reason for the increase of fracture strength and toughness.The number of black grain boundaries has an important influence on the increase rate of strength and toughness of diamond plates.The fracture strength of diamond plates with different number of black grain boundaries increases between 11%and 50%.This conclusion confirms that low-pressure/high-temperature heat treatment can improve the mechanical properties,and answers the controversial question of mechanical properties improvement more completely.