Types of Surface Coatings for Tooling
Surface Coatings for Tooling
In almost every type of production tooling, the most desirable feature to have is a very hard surface layer on a low strength but the tough body. Toughness is needed to survive mechanical shocks, that is, impact loading in interrupted cuts. Shocks occur in even continuous chip formation processes when the tool encounters a localized hard spot. The examples of such tooling include metal cutting tools, rock drill, cutting blades, forging
screws for extrusion of plastic and food products and sawmills and so on. Other applications, including parts for earth moving machinery, valves and valve seats for diesel engines, and many such parts involving high heat applications and in general, applications requiring wear resistance. Surface Coatings for Tooling
The various techniques employed for this purpose are discussed below:
1. Hard Facing This is a welding technique and has already been discussed in Chapter on “Welding Process”, under Art.
2. Nitriding Case Hardening : Discussed in Chapter.
3. Hard Chrome Plating : Hard chrome plating is done by the Electrolytic electroplating technique (See Art.). It is the most common process for wear resistance.
4. Flame Plating Flame plating is a process developed to prolong the life of certain types of cutting tools and for severe wear applications. By this process, a carefully controlled coating of tungsten carbide, chromium carbide (Cr, C2) or aluminum oxide is applied to a wide range of base metals. The more common materials which have been successfully flame – plated include aluminum, brass, bronze. Cast iron, ceramics, copper. glass, H.S.S., magnesium, molybdenum, nickle, steel and titanium and their alloys.
The process uses a specially designed gun into which is admitted metered amounts of oxygen and acetylene. A change, of fine particles of the selected plating mixtures, is injected into the mixture of oxygen and acetylene. Immediately after this, a valve opens to admit a stream of nitrogen to protect the valves during the subsequent detonation. The mixture is now ignited and an explosion takes place which plasticizes the particles and hurls them from the gun barrel at 750 m/s. The particles get embedded in the surface of the component and a microscopic welding action takes place, which produces a highly tenacious bond.
Each particle in the coating is elongated and flattened into a thin disc. The coating has a dense, fine – grain laminar structure with negligible porosity and an absence of voids or visible oxide layers. The layer of the plated material is about 0.006 mm, and this layer can be built up, by repeating the explosions, to thicknesses ranging from 0.05 to 0.75 mm, according to the requirements of any subsequent operations. The resultant coating is dense, hard and well bonded. Because of the hard dense structure of the coatings, flame – plating has provided the industry with a valuable tool for the solving of many abrasion, erosion and wear problems. For example, bushes for many applications, core pins for powder metallurgy, dies, gauges, journals, mandrels, and seals for high – duty pumps, have all been given much longer lives.
The process has influenced considerably certain types of cutting processes, especially in the glass, leather, paper, rubber, soap and textile industries and has proved to be of great advantage for components involving high heat applications such as “hot-end” of gas turbines. The coatings show an excellent resistance to galling and corrosion. Flame-plated coatings can be ground and lapped, if necessary. Resultant surface finish can be within the region of 0.025 Another advantage is that the components can be masked to enable the coatings to be placed precisely where required. The mixture of tungsten carbide coating material consists of cobalt ranging from 7 to l7% and the balance of tungsten carbide. Aluminum oxide plating mixture is almost of A103 (Above 99%). Chromium carbide plating mixture consists of about 75 to 85% of Cr3C, and balance of (Ni-Cr). for more related topics on mechanical engineering topics.