Investigation Of Cr-B-C Coating On Fe/Diamond Composites And Geological Drilling Bit

Energy,as a rigid demand for economic development,has become a bottleneck restricting the sustainable development of China's economy.How to obtain more usable and easy-to-use oil and gas resources on the existing land is an urgent problem to be solved.Drilling,as an important way to directly obtain oil and gas resources,is a scientific problem that we cannot bypass and must solve.As one of the important components of drilling tools,the correct selection of the drilling bit can achieve a multiplier effect with half the effort to complete a safe,efficient and economical drilling construction project.At present,the drilling industry in our country has gradually shifted to deep exploration,specifically in deep-hole hard rock formations.It is a common phenomenon that drilling bits are difficult to record,low drilling efficiency,long drilling cycles,and high drilling costs.With the continuous increase of drilling depth,the drilling strata become more and more complex,and the hard,compact,and weakly abrasive formation has always been a technical problem.The adaptability of the drilling bit to the formation determines the length of the drilling cycle and the level of drilling cost.With the continuous development of materials science,the selection of binder materials in the matrix of drilling bit has also changed from the past cobalt-based materials to copper-based materials and then to today's Fe-based materials.All of the Fe-based binders can become popular research objects nowadays owing to not only its low price but also the dual advantages of good wettability between cobalt-based binder and diamond and low sintering temperature of copper-based binder.However,Fe,as a catalyst for the allotropism conversion between carbon materials,can easily corrode diamond to form a brittle graphite interface layer under a preparation conditions of high temperature and low pressure.A drilling bit with good performance should ensure that the diamond can be edged out of the matrix at the right time,and this timing is closely related to the carcass'ability to hold the diamond.The ability of the matrix to hold the diamond is ultimately the strength of the interface between the binder and the diamond.Designing the interface composition and structure of the binder and diamond by coating the surface of the diamond particles is a direct and effective method to improve the diamond holding ability of the matrix and avoid the formation of the graphite interface layer.It can not only form a good coating effect with diamond,build a stable chemical bond,but also establish a reliable connection bridge with the matrix to effectively transfer stress,and make the matrix reach a certain strength.Thus,the blade can be released during the drilling process at appropriate time to increase the drilling speed and extend the service life.In this paper,the diamond surface is coated with boron carbide,chromium carbide and boron chromium carbide(Cr-B-C)coatings through high temperature solid phase synthesis and molten salt method.The composition of the boron carbide coating is B₄C,the composition of the chromium carbide coating is Cr₃C₂ and Cr₇C₃,and the composition of the boron carbide coating is Cr B,Cr₂B,Cr₃C₂ and Cr₇C₃.The synthesis temperature directly affects the composition and thickness of the coating on the diamond surface.For the boron carbide coating,the molten salt method is used to obtain a complete,uniform,and fully covered B₄C coating with a thickness of 2?m on the diamond surface when the synthesis temperature is 1200°C and the holding time is 6h.For the chromium carbide coating,the molten salt method can be used to synthesize coatings of different components when the synthesis temperature is 800?1100?and the holding time is 2h.With the increase of the synthesis temperature,Cr₇C₃ can be converted to Cr₃C₂,and the thickness of the chromium carbide coating is from 1.83?m to 4?m.For the boron chromium carbide coating,a two-step synthesis method is adopted.First,the B₄C coating is coated on the diamond surface by the high-temperature solid-phase synthesis method,and then the molten salt method is used when the synthesis temperature is 700?1200?and the holding time is 2h.With the increase of the synthesis temperature,the borides in the Cr-B-C coating are continuously converted to carbides with an accumulation of coating thickness from1?m to 8?m,and the Cr-B-C coating on the diamond surface can delay the conversion temperature of Cr₇C₃ to Cr₃C₂.The addition of the coating benefits to improve the oxidation resistance of diamond.Uncoated diamond can only withstand temperatures up to 700°C in the air,as a contrast,B₄C-coated diamond can hold 1090°C,and the Cr-B-C-coated diamond can endure 1151°C.The addition of the coating does not affect the static pressure strength of diamond,and has the effect of maintaining the static pressure strength of diamond before and after heat treatment.In this thesis,powder metallurgy was used to prepare Fe-based diamond composite materials containing boron carbide,chromium carbide and boron chromium carbide coatings and their corresponding geological impregnated diamond drilling bits at 850?and 30MPa for 5min.For Fe-based diamond composite materials,when the synthesis temperature of the chromium carbide coating and Cr-B-C coating on the diamond surface is 900?,1000?,respectively,it has the best physical and mechanical properties.The relative density of Cr-B-C coating(1000?)Fe-based diamond composite material increased by 2.63%,1.85%,and 1.76%,respectively,reaching99.34%,and the bending strength increased by 21.8%,15.7%,16.9%,respectively,to1405.6MPa,comparing with uncoated,boron carbide,chromium carbide coating(900?)diamond composite material,respectively.The Cr-B-C coating benefits to avoid the erosion of the Fe-based binder on the diamond,improves the interface bonding ability between the diamond and the Fe-based matrix,reduces the interfacial gap,compact the matrix,and improves the matrix's holding power to the diamond,thereby increasing the strength of diamond composites.The Cr-B-C-coated Fe-based impregnated diamond drilling bits have a slight increase in the mechanical drilling speed,while the service life is greatly increased,and the wear of the bits are reduced.Compared with conventional uncoated drilling bits,the life of Cr-B-C-coated drilling bits has increased by 33.2%while maintaining the drilling speed,and the wear per unit footage of the drilling bits has decreased by 9.6%.Compared with B₄C-coated drilling bits,the life of Cr-B-C-coated drilling bits has increased by 35.1%while the wear per unit footage of the drilling bits has been reduced by 57.1%.The increase of the life of the Cr-B-C coating drilling bits and the decrease of the wear extent are related to the increase of the interface adhesion between the diamond and the matrix.The Cr-B-C-coated Fe-based impregnated diamond drilling bits can appropriately increase the weight on bit during the actual drilling process,thereby increasing the drilling speed.The Cr-B-C coating can greatly enhance the interfacial adhesion of the matrix to the diamond,and improve the matrix's ability to hold the diamond.Under the premise that the strength of the matrix meets the conditions of use,an appropriate add in the weight on bit can increase the depth of the diamond in the drilling bit into the rock,improve the cutting efficiency,and enhance the drilling speed.