In this article we will discuss about the role of coatings on High Speed Steel (HSS) tools. Also learn about the process of deposition of coating on HSS tools in industries.
Role of Coatings on HSS Tools:
The performance of HSS tools can be enhanced many folds by applying coating of titanium nitride (TiN) on HSS metal cutting tools. This coating being golden in colour, also improves the appearance of HSS tool. HSS tool life is increased nearly ten times. Metal removal rate can nearly be double.
Before the HSS tool is totally consumed, more number of regrinds can be obtained. The preparation of substrate, the composition of coating and its method of deposition are very important to achieve real advantages of coated tool.
The coatings, now-a-days are applied by chemical vapour deposition techniques (cold process) or by physical vapour deposition (hot process). It has been observed that even partial removal of the microscopically thin coating by re-sharpening of tool does not eliminate the benefits of coating provided.
Even if coating is removed from the flank face of the tools, its existence on rake face contributes to reduced flank wear, though not as much as with coating remaining intact on flank face. Other coatings used on cutting tools are TiCN, TiAlCN and TiAIN.
A big advantage of inert coatings is the reduced adhesion wear. In fact, with coatings, the contact length at the chip-tool interface is substantially reduced which reduces the tool temperatures and consequently reduces adhesion wear near the cutting edge and also reduces the amount of superficial plastic deformation in the crater region. Because of the reduced adhesion and reduced tendency to form built up edge, surface finish improves considerably.
These coatings have been found to have a beneficial effect in reducing all the seven wear mechanisms, viz. adhesion, abrasion, fracture, wear by oxidation and subsequent adhesion, superficial plastic deformation, diffusion and solution wear, and plastic collapse of the cutting edge. This is so because the coatings reduce HSS tool temperatures and thus, also reduce the abrasion wear, the weakening of the tool material, and the plastic and superficial plastic deformation of the tool.
The endurance of uncoated and differently created tools with increase in feed is shown in Fig. 23.2. Initially with increase in feed rate the endurance of coated tools increases proportionately due to the work-hardening of the stainless steel during machining. Beyond certain limit the specific cutting force, relative to the rate of metal removal, diminishes. Under the lower stress loading, the endurance remains unchanged.
Processes of Deposition of Coating on HSS Tool:
In chemical vapour deposition method, the tools to be coated are loaded into a reactor chamber which is evacuated and filled with a carrier gas and precise quantities of reactive gases. The tools are then heated to a temperature of the order 950-1065°C. The high temperature causes the reactive gases to dissociate and thus the desired coating compound is formed on the tool surface.
To form TiN coating, titanium tetrachloride TiCl4 is used as the reactive gas and pure nitrogen or NH3 is used to supply nitrogen. HCl gas formed during the process is neutralised properly. As the tempering temperature of HSS is much lower than temperature encountered in this process, these tools after treatment are restored to proper condition by a vacuum heat treatment.
In physical vapour deposition technique, the driving force is not the high temperature but the ion bombardment. Temperature is of the order of 260-480°C and process is carried under vacuum. Deposition could be done by sputtering, or reactive ion plating, on arc evaporation.
In sputtering process, a high voltage is applied between the tool (anode) and titanium (cathode). A reactive gas (nitrogen in the case of TiN coating) is supplied. Cathode supplies the coating ions liberated by the bombardment of the gas ions.
To speed up the process properly shaped magnetic field can be applied. In the reactive ion plating process, the vaporised target of Ti (obtained by evaporating molten pool of metal by means of an electron beam gun) is ionised in the presence of a reactive gas (N2 in case of TiN coating).
An electrical potential, is applied to accelerate the Ti ions toward the tool, where the TiN combines and is deposited as a coating. In the arc evaporation process, the controlled arcs are moved over the solid surface of the target to generate minute sources of vapour containing a high ion content.
The metallic ions are accelerated toward the tool by applying an electrical potential and using nitrogen gas to form desired compound of TiN. Coatings of 2 to 2.5 micron can be applied by this process whereas thick coating (7.5 micron) is applied by chemical vapour deposition.
It is a relatively fast process. This process produces surface finish of coating same as of basic tool surface, whereas chemical vapour deposition produces matter surface which may require polishing for some applications. While chemical vapour deposition technique deposits, TiN uniformly all over, physical vapour deposition techniques deposit it only on surface in front of bombarding ions.
Titanium carbide coating is found useful with tools for metal forming applications (punches and dies etc.) TiC coating in preferred at lower speed where abrasion is main cause of wear. Research for depositing boron nitride is going on.
TiN is valuable tool coating because of following reasons:
(i) It permits tight adhesion of HSS.
(ii) It imparts lubricity and resistance to built up edge.
(iii) Its hardness is 80-85 Rc in comparison to 65-70 for hardened HSS.
(iv) It offers a low coefficient of friction with virtually all the metals machined.
(v) It possesses high chemical stability, being extremely resistant to corrosion and chemical reaction with common work piece materials.
(vi) It retains all desirable properties at high temperatures encountered during machining.
(vii) 2.5 micron thick coating enhances the abrasion resistance of tool, resisting adhesion, welding, galling, cratering and formation of built-up-edge. Cutting forces and spindle power are reduced. Tool life is increased and high speeds and feeds are permitted.