Tham khảo tài liệu 'cutting tool materials part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Cutting Tool Materials 21 o Figure 11. A typical super-glide coating of molybdenum disulfide M0S2 applied to a hard-coating on a tool s substrate - weak bonds between crystal layers allow easy movement of the planes. Courtesy of Guhring Moreover ceramic tools at this juncture were only really employed for turning operations and in particular in stable machining where interrupted intermittent cutting operations did not occur. With the recent advances in powerful and very rigid CNC machine tools this has opened-up the possibility of utilising ceramic tooling either in a purely sintered monolithic tooling insert or more recently as a multi-coated variant - more will be said on this topic shortly. Returning to the monolithic ceramic cutting tool materials they have normally been available in three distinct grades which will now be mentioned. These cutting inserts consist of Pure ceramic - this is the traditional tooling insert material consisting of aluminium oxide. The alumina is white in colour and is produced by cold pressing powder in the desired insert geometry dies8 with subsequent sintering the fused alumina particulates are sintered together thereby significantly decreasing porosity. These ceramics have been known in the past as pure oxide or cold-pressed ceramics. The major disadvantage of such ceramics is their low thermal conductivity making them highly susceptible to thermal shock . the hot and cold thermal cycles that can occur when interrupted cutting takes place . These thermal shock 8 These consolidated cutting inserts produced in compound or floating die sets from the admixed powders are termed green compacts and are friable that is having very limited mechanical strength and must be gently handled prior to sintering - thereafter the desired mechanical strength occurs. 22 Chapter 1 problems are exacerbated by short machining cycle times variable depths of cut and higher machining speeds. Pure alumina inserts can be improved by additions of zirconia
