Advanced Surface Enginering – A Current Review

By Max Babiwww.maxbabi.com

Advanced Surface Engineering has made enormous progress in the last three decades or so. From a humble beginning in the early 1900s when the Swiss inventor Max Ulich Schoop put together an elementary metal spray unit, to the extra-sophisticated vacuum-based surface modification treatments, it has been a century full of innovations indeed.

Surface Engineering as a science and technology encompasses a breath-taking gamut of processes right from the very humble electro-deposition of metals e.g. chrome or nickel plating up to the very elaborate PVD and plasma assisted surface modification treatments today. We would do well to remember that even the humble processes like gas metal spraying, one of the most accessible and really economically effective coating processes or the even more humble electro-deposition have benefited greatly in recent times. Exactly how? Well, laser beams have been shone into the humble electrolytic baths to speed up the deposition and microprocessor controlled powder delivery systems in gas metal spraying ensure that even the super-sophisticated ‘funcitionally gradient matierlas’ or FGMs can also be sprayed. Even composite composition can be sprayed with computerized controls.

Due to space constraints we will not be doing justice to every variant of note here in this introductory review of the ASEs. Thus primary focus of these notes, being compiled as a companion reader to the author’s powerpoint presentation, will focus primarily only on Thermal Spray techniques and plasma ion nitriding group of ionic treatments.

Thermal Spray family of material spraying techniques :

What is Thermal Spray?

Thermal spray coating involves the use of a torch to heat a material, in powder or wire form, to a molten or near-molten state, and the use of a gas to propel the material to the target substrate, creating a completely new surface.  The ‘torch’ or gun may be hand-held, or machine-mounted, it may work with the very familiar heat source i.e. gas flame combustion or arc-spray using an electric arc as the heat source. Plasma spray also uses the electric arc fundamentally as the heat source, but it is a ‘plasma’ device in the sense the formation of an

arc plasma becomes the source of very high temperatures, far above the range of electric arc as a heat source. Temperatures as high as 70,000 to 100,000 degrees Celsius are not uncommon.Schematic diagram below illustrates how a typical thermal spray system works.

Thermal spray coatings are a highly cost-effective and straight-forward method for adding superior properties and performance qualities to a given engineering surface. 

What Are The Different Techniques : 1. Combustion powder, 2. Combustion wire, 3. Electric wire arc, 4. HVOF, 5. Plasma processes atmospheric (APS) 6.vacuum (VPS, LPPS, LVPS) conditions.The coating material may be a single element, alloy or compound with unique physical properties that are, in most cases, achievable only through the thermal spray process. Coatings to extend product life, increase performance and reducethe production and maintenance costs.Benefits of thermal spray are:• Resist Wear• Retard Corrosion • Control Clearances.

An example of restoration

1 - Casting and machining,

2 - Surface activation, 3 - Coated cylinder bore, 4 - Finish machining (diamond honing)

The variety of products and coatings that can be enhanced by thermal spray is virtually limitless. The coatings can be metallic, ceramic, plastic or any desired combination to meet a range of physical criteria.

• Master Fluid Environments • Salvage Worn Components• Conquer High Temperatures •Enhance Electrical Properties • Dimensional restoration, • Maintaining precise clearances, etc.

Roth OEM parts and salvaging/recycling work is possible.

Salvaged parts can be better than original.

Plasma Coated Automotive Cylinder Bores :• Cut Costs, Reduce Friction, Improve Fuel Use, Last Longer.• Meet cost and weight constraints,• Meet Fuel economy standards.• The auto industry has been moving away from heavy cast iron engine blocks to lighter aluminum alloys (AlSi).

Plasma Coated Automotive Cylinder Bores :

• The residual porosity of the plasma coating helps to significantly reduce the coefficient of friction through a micro-cavity lubrication system. • Wear resistance of these coatings is an order of magnitude better than cast iron and the engine block is much more predictable. Plasma Coated Automotive Cylinder Bores :

• The main challenge of an industrial solution for the coating of cylinder bores is not only to develop a suitable material and process, but also to integrate that process into a cost-effective, high-volume, fully automated production coating system. •Coatings Designed to Beat Friction:Cylinder bore, piston and piston ring friction represent nearly 40% of the mechanical power losses in the engine. Using extensive R&D knowledge, Sulzer Metco has produced.•a variety of low-friction coatings for aluminum / silicon (AlSi) engine cylinder bore applications. The coatings exhibit:•Low coefficient of friction •Superior bonding to the AlSi bore material • Thermal shock resistance • Scuff resistance • Lower wear rate vs. cast iron • Reduction of oil consumption•Fully automatic spray facilities, , can be integrated into any production setup—in line or cell type. • Prototype engines within the industry using cylinder bore coating systems were operated for up to 400,000 km (250,000 miles) without need of repair. –Sulzer Metco. •Following on this example, thousands of production cars have been manufactured. • This technology works for gasoline as well as diesel engines. • Sulzer Metco also applies this technology to high performance race cars, go karts and motor bike engines.

Thermal Spray : The Electric Arc Wire Spray Process

Thermal Spray : Plasma Spray Process

-It is the spraying action of molten or heat softened material onto a surface to provide a coating. -- Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. -- The hot material impacts on the substrate surface and rapidly cools forming a coating.

Plasma spray in action.

Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconia unlike combustion processes. Plasma sprayed coatings are generally much denser, stronger and cleaner than the other thermal spray processes with the exception of HVOF and detonation processes.

Plasma spray coatings probably account for the widest range of thermal spray coatings and applications and makes this process the most versatile.Disadvantages of the plasma spray process are relative high cost and complexity of process. Plasma/ion nitriding

This technique is now being used worldwide for controlled surface hardening of steels, mainly.Using a vacuum reactor, this process offers cleanest solutions to surface hardening with minimum mechanical distortion or metallurgical damage.Here, components are hardened by nitrogen diffusing into the matrix and in final stages, a very precise formation of a nitride layer as desired.Plasma/ion nitriding is far more superior to gas nitriding an all types of liquid bath processes. It is up to 10 times faster, much more precise, and eliminates the formation of a ‘white layer’. Parts PIN’d can be used straight-away without any machining or finishing.

It consumes less power, offers quicker returns and produces no pollution at all.

Plasma/ion nitriding technique produces uniform surface hardening without affecting bulk hardness.Tailor-made layers can be achieved, for example the type of nitride chemistry can be controlled.For friction only applications –an epsilon type nitride works well, whilst for impact involving uses, a gamma prime layer works better. Plasma/ion nitriding technique produces uniform surface hardening without affecting bulk hardness.Tailor-made layers can be achieved, for example the type of nitride chemistry can be controlled.For friction only applications –an epsilon type nitride works well, whilst for impact involving uses, a gamma prime layer works better. Applicable to broadly three types of parts:-1.Dies/toolings e.g. plastic moulding, pressure die casting type, sheet metal processing type.

2.Tools (e.g. metal cutting tools) e.g. milling cutters, shaper cutters, hobs, broaches, etc.3.Industrial components : crankshafts, camshafts, gudgeon pins, sheet-metal parts,

Plasma nitrided parts:

Both ‘Plasma Oxy-nitriding’ and post-oxidation of plasma nitrided parts are popular abroad.Instead of running a separate ’steaming treatment’ for nitrided part, sometimes a gas mixture with controlled amounts of oxygen can be supplied in the last phase of nitriding.

A picture of glow discharge surrounding parts being plasma nitrided

Plasma oxy-nitrided automotive part.

Advanced Surface Engg. Has exploded into a hundred variants in the past few decades. Duplex Treatments are now practically used in commercial environments.PVD group of treatments, including the exciting world of plasma enhanced treatments like PACVD – PECVD etc., are also now common.

Indian scenario could do well to get into the higher end, capital intensive equipment like Laser Beam processing, ion implantation, etc. which are no longer confined to laboratories in the industrially advanced countries.Perhaps global competition may force us to rethink.

Indian scenario could do well to use automation e.g. fully computerized production centres for OEM parts . Internet-based satellite control over local coating centres is being offered by specialists in Euro-capitals. Advanced Japanese technologies are yet to find their feet in India. Local talent remains untapped.

-end.