The Application Status for Coating of Cutting Tools and Its Development Trends
The cutting tools’s coating has been developed to multi-layer and gradient layer from singer layer,in which the TiN,TiCN,TiAlN,AltiN,CrN and TiAlCN are the traditional layers.So far,the nano multi-layer and nano structure such as CrTiAlN,TiAln/SiN,CrAlN/SiN and CrTiN has gained ground further .In the future ,the multi-layer composite structure coating will become more and more important and prevalent.
The multi-layer composite structure coating is formed by deposition of three layers ,in which the bottom layer is thin but hard adhesion layer ,and the middle is AlTiN step-ladder type core layer consist of single layer or the step-ladder structure layer,the top layer is nano-structure which is as hard as 50GPa with very high thermal-resistance property ,even under 1200℃,it still works stably .
Consequently ,the multi-layer composite structure coating has been the mainstream of development ,however ,how we should select the proper one due to its diversity,which mainly depends on its actual application.For example ,the milling tools are very suitable to take the multi-layer composite structure coating ,as it is very helpful to reduce cracking cases in processing .
Coating technology is the main way to improve the cutting tools’ performance,by means of it ,the cutting tools’ wear-resistance capability is increased ,also prolonged its service life,meanwhile ,the accuracy of surface of parts to be machined is improved ,and risen the cutting and feeding speed, hereby ,expedite metal-cutting efficiency finally .Today ,tungsten carbide as the mainstream of cutting tools’ material,coated ones take 80%,in which CVD occupies 60%~65%,and PVD takes the rest .
With respect to the CVD coating ,including TiCN,TiC,TiN,ZrCN and Al2O3 etc are the compounded multi-layer composite coating which are very positive and favorable in improving the overall performance of the coating ,such as binding strength ,toughness,wear-resistance and abrasion-resistance. At present ,the typical multi-layer structure coating consist of TiN(top layer)+Al2O3(middle layer)+TiCN(bottom layer) is under meliorating on coating process and thickness .The MTCVD(medium temperature chemical coating) could minimize brittle η phases aroused in the boundaries between the coating and substrate due to its comparatively low technique temperature and fast deposition speed ,while reducing the tensile crack commonly happened in the high temperature CVD coating process .Thereby ,the TiCN coated by MTCVD technique has become the main composition of CVD multi-layer structure ,meanwhile it has been used for α- Al2O3 coating such as ISCAR’s α-IC9150、α-IC9250、α-IC9350 and α-IC4100 ,by means of it ,improving the bonding strength of coating and substrate ,flank and front wear-resistance ,and anti-adhesion capability .
With regard to the PVD coating ,it also has been developed from single layer such as TiN ,TiCN TiAlN to composite layers that is hard coating + soft coating.In order to achieve high cutting speed and dry-cutting requirements ,the red-hardness of coated cutting tools has been become the hot point in PVD technology development in recent years .The coating of AlTiN ameliorated from TiAlN has better red-hardness, chemical stability and anti-oxidation performance by increasing the Al(over 50% Al) ingredient in the film,like as ISCAR’s Al-IC910(machining cast-iron and steel),Al-IC900 ,Al-IC930(machining steel,stainless steel,harden steel,cast-iron,high temperature alloys)
The one of the most important characters of cutting tools’ coating is composite structure currently,by means of composition of coating materials,coating layers ,CVD and PVD ,such as ISCAR’s DT7150(K05~K25) composed with MTCVDAL2O3 and PVD TiALN has very high overall performance using for machining gray cast iron and nodular cast iron .On the contrast ,diversity is another trend of cutting tools’ coating,there are variety of nitride ,oxide coating materials ,as well as TiB,SN coating ,diamond costing ,PCBN coating etc .The underlying causes for the diversity is professionalism that means to adopt different coatings for different needs, and undertake control and change largely on the coating composition ,percentage ,structure and thickness,so as to get used to different machining materials and cutting condition ,and hereby improve the cutting tools performance apparently .Like as the CrAlN coating,which take Cr replace of Ti element ,has hardness as HV3200 and 1100℃ oxide temperature,comparing to the TiAlN,it has better tensile property and is more capable to take Intermittent cutting and machine materials which very difficult to process. If replace the Al element with Si in TiAlN ,we can have the TiSiN coating to take hard cutting work,also can have the CrSiN with good lubricant property which is very capable to machine aluminum and stainless steel that are the material with strong adhesion property .In addition,micro-granular of the coating materials is another trend for the modern cutting tools,nano-strucure composite coating more and more prevalent .In the future ,the cutting tools’ coating is a systematic conception that means the coatings have to be systematically combined according to the changing application condition of the modern cutting tools ,it is a more complicated and systematic project comparing to the traditional conception that coating a film on the substrate .
