Study On The Design,preparation And Cutting Behavior Of CVD Coatings On Cermet Inserts

Cermets are widely used in the preparation of cutting tools and wear parts due to their low density,low cost,high hardness and high wear resistance.Ti（C,N）-based cermets use our resource-rich Ti as the main component,while saving the scarce strategic resource W,which has significant resource and cost advantages compared to cemented carbide.With the emergence of difficult to machine materials,cemert cutting inserts are susceptible to adhesive wear during the cutting process,resulting in a shortened service life.The requirements for high service life,high wear resistance and high efficiency of cutting tools under dry cutting conditions can no longer be met by simply strengthening the cermet substrate.In order to improve the service life and cutting quality of cermet cutting inserts,surface coating of cermet inserts is essential.Although the coating has high wear resistance and can improve the service life of cermet cutting inserts,the composition,microstructure and physical and mechanical properties between the coating and the cermet substrate differ greatly.When CVD coating is applied to the cermet surface,there are problems of high residual thermal stress in the coated ceremt,low interfacial adhesion strength and a large number of defects（voids and cracks）are very likely to appear in the coating.To this end,this paper proposes to address these issues through the design of low-stress coating structures,the preparation of dense columnar crystal transition layers,the control of coating growth processes and orientation,the control of cermet substrate characteristics and post-treatment of coatings.This paper first designed a low-stress MT-Ti CN/α-Al₂O₃/Ti N multilayer coating structure based on the interfacial stress distribution characteristics of coated cermet inserts,secondly studied the influence of the deposition process of the transition layer and functional layer on the structure and mechanical properties of the coated cermet,subsequently controlled the composition and microstructure of the cermet matrix and studied the influence of the matrix on the growth mechanism of the coating nucleation.The MT-Ti CN/α-Al₂O₃/Ti N coated cermet cutting inserts with low stress,high bond strength,high wear resistance and oxidation resistance were prepared by sandblasting post-treatment to further regulate the surface stress state and their cutting behaviour was investigated.The main research contents and conclusions in the paper are as follows:（1）Based on the characteristics of the interfacial stress distribution of the coated cermets,the structure of the transition layer and the functional layer was optimised.The finite element simulation results show that the residual stress of the coated cermets is relatively low when the coating deposited on the surface of the cermets is Ti CN,and the thermal stress of the coated cermets increases and then decreases with the increase of the thickness of the Ti CN coating.The interfacial stress distribution of the coated cermets indicates that the adhesion strength of the coated cermets is favoured when the thickness of the Ti CN coating is chosen to be≥5μm.For the multilayer coating system Ti CN（≥5μm）+α-Al₂O₃（>3μm）+Ti N 1.5μm,the stress in the coated cermet gradually decreases as the thickness of the alumina coating increases.To ensure a strong bond between the coating and the cermet substrate,the optimal range of thermal expansion coefficient of the substrate is 7.5～9.5×10^-6/K and the elastic modulus should be as small as possible.（2）The effect of the deposition process of the transition layer and the functional layer on the structure and mechanical properties of the coated cermets was investigated.HT-Ti CN coatings have a fine isometric grain+porous structure,while MT-Ti CN coatings have an elongated columnar crystal structure,and MT-Ti CN coatings have higher bond strength,hardness and H/E than HT-Ti CN coatings.The（300）orientation of the alumina coatings was obtained by varying the CO₂ content of the oxide layer deposition atmosphere,but the texture coefficients did not differ,due to the fact that the CO₂ content changed the oxidation potential of the titanium-containing complexes during the preparation of the transition layer.By varying the flux of CO in the deposition atmosphere of the oxide layer,（001）strongly texture alumina coatings can be obtained.The Ti CN coatings deposited on the cermet surface are all elongated fibrous columnar crystals,while the oxide layer deposited on the MT-Ti CN surface consists of a number of fine nanocrystalline grains stacked on top of each other.The growth rate of the alumina coating can be changed by varying the flow rate of CO in the oxide layer deposition atmosphere,which can promote the formation of alumina in different textures.At a CO flow rate of 1 L/min,the alumina has a（001）strong texture and the best bond strength,hardness,modulus of elasticity and H/E of the coated cermet.（3）The effect of cermet matrix composition on the nucleation growth,microstructure and properties of the coatings was investigated.The addition of tungsten carbide to the cermet matrix refines the hard phase,and the fine-grained hard phase provides more nucleation sites for the nucleation growth of the coating.With the increase of tungsten carbide content in the matrix,the texture coefficient of（111）crystalline surface in Ti CN coating gradually increases from 0.93 to 1.42,which is attributed to the increase of fine grains with core-rim structure in the matrix,which increases the surface energy of Ti CN coating during nucleation growth.The Ti CN coating on the surface of the cermet substrate grows epitaxially semi-coherent on the core-ring phase and non-coherent nucleation on the binder phase,and the growth rate on the core-rim hard phase is higher than that on the binder phase,which makes the coating form fine nanocrystals on the binder phase first,and these nanocrystals ensure the adhesion strength of the coating to the substrate and reduce the residual stress of the coating,while the Ti CN coating on the surface of the hard phase is nucleated and grows into fine fibrous columnar crystals.（4）The effect of the sandblasting process on the surface condition and performance of coated cermet cutting inserts was investigated.Sandblasting pressure and blasting time have a significant influence on the degree of surface erosion of coated cermet inserts.The surface roughness of the unblasted coating is high and there are large residual tensile stresses on the surface of the coating,while the surface roughness of the coated cermet inserts is significantly reduced after blasting,the residual stresses on the surface gradually change from tensile to compressive stresses and the surface hardness is increased.（5）The cutting behaviour of Ti CN/α-Al₂O₃/Ti N coated cermet inserts was investigated.The multilayer coatings deposited on the cermet substrate all have a columnar crystal structure,but the growth texture of each coating is different.The flank wear morphologies show that the growth texture of（220）is more wear resistant than（111）in the Ti CN coating,and the single orientation is more wear resistant than the multiple orientation in theα-Al₂O₃ coating.The wear morphology of coated inserts shows that the surface morphology of the coating is related to the structural design and preparation process of the coating,and its surface not only affects the friction coefficient between the insert and the chip,but also affects the temperature distribution at the chip-insert contact interface.In addition,the selection of the functional layer of the coating is very important for the design and application of coated tools,especially with regard to the thickness and thermophysical properties of the coating.The wear characteristics and formation mechanisms of strongly texture coated cermet cutting inserts were investigated.The CD coated cermet inserts with（001）strong orientation are comparable to commercial CCX and MC5015 in terms of rake wear morphology,flank wear morphology,cutting forces and cutting temperatures.During cutting,the strongly texture coated cermet inserts exhibit wear characteristics such as BUE,groove wear,coating flaking and micro chipping,with the root cause of failure being caused by abrasive and adhesive wear.The mechanism for the formation of nano-scale grooves on the coating surface is that when a high temperature chip flows across the coating surface,the high temperature chip has an etching effect when it comes into contact with the raised part of the coating grain,resulting in a stepped,low energy surface.