Study On Etching Of Synthetic Diamond Crystallites By Iron Group Metals And Iron-group Metal Salts

Synthetic diamond possesses outstanding properties,such as superior hardness,high wear resistance,and strong chemical inertness,thereby making them preferable as abrasives for machining hard and brittle material s.Given the small size of the diamond crystallites,bind er materials are usually required to bond them together to form diamond tools with specific size,shape,and strength.However,due to the smooth surface and high surface energy,it is difficult for binders to wet diamond during tool fabrication process.Consequently,they are combined by mechanical joint,and the adhesion strength is quite weak.During machining process,diamond grains prematurely fall off from the tool surface,resulting in the non-machining loss.To improve the adhesion strength between diamond and binder materials,the present adopted methods are surface plating or coating on diamond.Due to the limitations of the these surface treatment technologies,iron group metals and iron-group metal salts were used to etch diamond crystallites bas ed on the thermochemical etching in this thesis.Influences of different parameters on diamond etching by iron group metals were systematically studied.Combined with the thermodynamic calculation,the dominant etching mechanism of diamond by iron group metals was discussed.When iron-group metal salts were used as catalysts,the effect of temperature on diamond etching and the mechanism were investigated.To validate the application of surface etched diamond,diamond saw blades were fabricated by mixing di amond treated using different methods with Cu-based or Fe-based metal bond,and the mechanical properties and sawing performances were compared.The main research results are as follows.(1)The effect of temperature on the diamond etching by iron group me tals is investigated emphatically.Results show that temperature plays a key role in diamond etching process.The initial tem peratures of diamond etching by iron,nickel and cobalt are 800°C,700°C and 600°C,respectively.The extent of diamond etching increases with the temperature,and at the same processing temperature,the etch area and depth on {100} planes are larger than those on {111} planes.W hen iron is used as the catalyst,etching on diamond {100} planes occurs along the normal direction;while on {111} planes etching originates from the edge of crystal plane and then extends to the centre.When nickel and cobalt are used as the catalysts,normal etching dominates on both {100} and {111} planes,and reversed pyramids and hexagons are formed after etching.Compared with nickel,the uniformity of diamond etching by cobalt is better,and the etch area and depth are larger.(2)Influences of holding time,the ratio of metal power to diamond and the metal particle size on diamond etching by iron group metals are studied systematically.With the extended holding time,the area and depth of etch pits on diamond surface etched by nickel and cobalt increase.However,after the holding time exceeds a certain value,its effect on diamond etching decreases.W hen the weight ratio of cobalt power to diamond reduces,the etch area and depth on diamond {100} and {111} planes also decrease.Meanwhile,etching on diamond surface becomes nonuniform.Moreover,with the increase of powder size of cobalt,the extent of diamond etching mitigates,and the the uniformity becomes worse.(3)The mechanism of diamond etching by iron group metals and the formation law of etch pits are discussed.Diamond etching process by iron group metals can be described as below: At lower temperatures,solid metal powder establishes a mechanical contact with the surface of diamond.An increase in temperature causes gradual melt of metal particles and dissociation of hydrogen terminated on the dangling bonds.As a result,the contact conditio n between metal and diamond changes.Upon wetting by the molten metal,diamond tends to undergo a phase transformation from its stable sp3 form to sp2 when it is energetically favourable.Subsequently,under the driving force of concentration,the newly formed graphic carbon at the interface of diamond and metal gets transported through the molten metal along the direction far away from the interface.The theoretical etch depth of diamond by iron is calculated according to the Fick laws,which is generally consistent with the measured P-V values.Being similar to the step-growth mode of diamond,diamond etching by iron group metals proceeds by layers.The bottom of etch pits corresponds to the etched crystal plane,and sidewalls of etch pits are composed by adjacent planes of the etched plane or its more stable secondary adjacent crystal planes.(4)The etch behavior and mechanism of diamond etching by iron oxalate dihydrate,cobalt oxalate dihydrate and cobalt nitrate hexahydrate at different temperatures are explored.With elevated temperature,the extent of diamond etching increases.Especially when diamond is etched by iron oxalate dihydrate,diamond is severely etched after temperature exceeds 900 °C.Etch pits can be fabricated on diamond {100} and {111} planes simultaneously by cobalt oxalate dihydrate and cobalt nitrate hexahydrate.However,compared with cobalt powder,diamond etching by cobalt oxalate dihydrate or cobalt nitrate hexahydrate is nonuniform.Although on the same plane,the size and depth of etch pits differ a lot.Diamond etching mechanism by iron oxalate dihydrate is graphitization and oxidization of diamond,and the dorninant etching mechanism of diamond by cobalt oxalate dihydrate or cobalt nitrate hexahydrate is graphitization.(5)Mechanical properties of diamond treated using different methods and their metal bond sintered bodies are analyzed.Compared with with the untreated diamond,the single grain compression strength and impact toughness of Ti-coated diamond are larger.After the sintered body of Ti-coated diamond and Cu-based bond is fabricated,the hardness,bending strength and impact strength of the sintered body do not show any significant changes compared to the untreated diamond.However,the bending strength and impact st rength of the sintered body of Ti-coated diamond and Fe-based bond decline slightly.Although the single grain compression strength and impact toughness of Co-etched diamond are smaller,the bending strength and impact strength of sintered bodies of Co-etched diamond and Cu-based or Fe-based bond are obviously improved.(6)The sawing performances of different diamond saw blades are compared.In contrast the untreated diamond saw blade,the diamond protruding height on the working surface of Ti-coated diamond Cu-based bond saw blade varies little,but the percentage of 50% protruding height shows slight increase.After the sawing experiment,the fall-off rate of diamond descends,and the service life of Ti-coated diamond Cu-based bond saw blade is improved b y 15%.The diamond protruding height and the percentage of 50% protruding height on the working surface of Ti-coated diamond Fe-based bond saw blade rise,and the fall-off rate of diamond experiences dramatic decline.The sharpness of Ti-coated diamond Fe-based bond saw blade is poor,but its service life is expanded by 11%.As for Co-etched Cu-based or Fe-based bond diamond saw blades,the diamond protruding height and the percentage of 50% protruding height are larger,and the fall-off rate of diamond decrease sharply.The self-sharpness of two kinds of saw blades is good,and their service lifes are prolonged by 12% and 8%,respectively.