Briefly introduce the processes of surface engineering
China precision machining The processes of surface engineering, or surface treatments, tailor the surfaces of engineering materials to:
(1) control friction and wear,
(2) improve corrosion resistance,
(3) change physical property, e.g., conductivity, resistivity, and reflection,
(4) alter dimension,
(5) vary appearance, e.g., color and roughness,
(6) reduce cost. 。
Common surface treatments can be divided into two major categories: treatments that cover the surfaces and treatments that alter the surfaces.
１.Covering the Surface
The treatments that cover the surfaces include organic coatings and inorganic coatings.
China precision machining The inorganic coatings perform electroplatings, conversion coatings, thermal sprayings, hot dippings, furnace fusings, or coat thin films, glass, ceramics on the surfaces of the materials.
Electroplating is an electrochemical process by which metal is deposited on a substrate by passing a current through the bath.
Usually there is an anode (positively charged electrode), which is the source of the material to be deposited; the electrochemistry which is the medium through which metal ions are exchanged and transferred to the substrate to be coated; and a cathode (negatively charged electrode) which is the substrate to be coated.
Plating is done in a plating bath which is usually a non-metallic tank (usually plastic). The tank is filled with electrolyte which has the metal, to be plated, in ionic form.
The anode is connected to the positive terminal of the power supply. The anode is usually the metal to be plated (assuming that the metal will corrode in the electrolyte). For ease of operation, the metal is in the form of nuggets and placed in an inert metal basket made out non-corroding metal (such as titanium or stainless steel).
The cathode is the workpiece, the substrate to be plated. This is connected to the negative terminal of the power supply. The power supply is well regulated to minimize ripples as well to deliver a steady predictable current, under varying loads such as those found in plating tanks.
As the current is applied, positive metal ions from the solution are attracted to the negatively charged cathode and deposit on the cathode. As a replenishment for these deposited ions, the metal from the anode is dissolved and goes into the solution and balances the ionic potential.
China precision machining Thermal spraying process. Thermal spraying metal coatings are depositions of metal which has been melted immediately prior to projection onto the substrate. The metals used and the application systems used vary but most applications result in thin coatings applied to surfaces requiring improvement to their corrosion or abrasion resistance properties.
Thermal spray is a generic term for a broad class of related processes in which molten droplets of metals, ceramics, glasses, and/or polymers are sprayed onto a surface to produce a coating, to form a free-standing near-net-shape, or to create an engineered material with unique properties.
In principle, any material with a stable molten phase can be thermally sprayed, and a wide range of pure and composite materials are routinely sprayed for both research and industrial applications. Deposition rates are very high in comparison to alternative coating technologies.
Deposit thickness of 0.1 to 1mm is common, and thickness greater than 1cm can be achieved with some materials.
The process for application of thermal spray metal is relatively simple and consists of the following stages.
(1) Melting the metal at the gun.
(2) Spraying the liquid metal onto the prepared substrate by means of compressed air.
(3) Molten particles are projected onto the cleaned substrate.
There are two main types of wire application available today namely arc spray and gas spray.
ARC—A pair of wires are electrically energized so that an arc is struck across the tips when brought together through a pistol. Compressed air is blown across the arc to atomise and propel the autofed metal wire particles onto the prepared workpiece.
GAS—In combustion flame spraying the continuously moving wire is passed through a pistol, melted by a conical jet of burning gas. The molten wire tip enters the cone, atomises and is propelled onto the substrate.
Thin-Film Coatings. Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are two most common types of thin-film coating methods.
PVD coatings involve atom-by-atom, molecule-by-molecule, or ion deposition of various materials on solid substrates in vacuum systems.
Thermal evaporation uses the atomic cloud formed by the evaporation of the coating metal in a vacuum environment to coat all the surfaces in the line of sight between the substrate and the target. It is often used in producing thin, 0.5μm, decorative shiny coatings on plastic parts.
The thin coating, however, is fragile and not good for wear applications. The thermal evaporation process can also coat a very thick, 1mm, layer of heat-resistant materials, such as MCrAIY—a metal, chromium, aluminum, and yttrium alloys, on jet engine parts.
Sputtering applies high-technology coatings such as ceramics, metal alloys, organic and inorganic compounds by connecting the workpiece and the substance to a high-voltage DC power supply in an argon vacuum system.
The plasma is established between the substrate (workpiece) and the target (donor) and transposes the sputtered off target atoms to the surface of the substrate.
When the substrate is non-conductive, e.g., polymer, a radio-frequency (RF) sputtering is used instead. Sputtering can produce thin, less than 3μm (120μin), hard thin-film coatings, e.g., titanium nitride (TIN) which is harder than the hardest metal.
Sputtering is now widely applied on cutting tools, forming tools, injection molding tools, and common tools such as punches and dies, to increase wear resistance and service life.
CVD is capable of producing thick, dense, ductile, and good adhesive coatings on metals and non-metals such as glass and plastic. Contrasting to the PVD coating in the “line of sight”, the CVD can coat all surfaces of the substrate.
Conventional CVD coating process requires a metal compound that will volatilize at a fairly low temperature and decompose to a metal when it contacts with the substrate at higher temperature.
The most well known example of CVD is the nickel carbonyl (NiCO4) coating as thick as 2.5mm (0.1in.) on glass windows and containers to make them explosion or shatter resistant.
Diamond CVD coating process is introduced to increase the surface hardness of cutting tools. However, the process is done at the temperatures higher than 700℃ (1300℉) which will soften most tool steel.
Thus, the application of diamond CVD is limited to materials which will not soften at this temperature such as cemented carbides.
Plasma-Assisted CVD coating process can be performed at lower temperature than diamond CVD coatings. This CVD process is used to apply diamond coatings or silicon carbide barrier coatings on plastic films and semiconductors, including the state of the art 0.25μm semiconductors.
２. Altering the Surfaces
The treatments that alter the surfaces include hardening treatments, high-energy processes and special treatments.
High-energy processes are relatively new surface treatment methods. They can alter the properties of surfaces without changing the dimension of the surface. Common high-energy processes, including electron beam treatment, ion implantation, and laser beam treatment, are briefly discussed as follows:
Electron beam treatment. Electron beam treatment alters the surface properties by rapid heating—using electron beam and rapid cooling—in the order of 106℃/see in a very shallow region, 100μm, near the surface. This technique can also be used in hardfacing to produce “surface alloys”.
Ion implantation. Ion implantation uses electron beam or plasma to impinge gas atoms to ions with sufficient energy, and embed these ions into atomic lattice of the substrate, accelerated by magnetic coils in a vacuum chamber. The mismatch between ion implant and the surface of a metal creates atomic defects that harden the surface.
Laser beam treatment. Similar to electron beam treatment, laser beam treatment alters the surface properties by rapid heating and rapid cooling in a very shallow region near the surface. It can also be used in hardfacing to produce “surface alloys”.
China precision machining The results of high-energy processes are not well known or very well controlled. But the preliminary results look promising. Further development is needed in high-energy processes, especially in implant dosages and treatment methods.