Nikasil® and Alusil a Primer on Some of the More Common Specialty Coatings and Cylinder Materials Found in Small Displacement Engines
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EP 10-2008 18-32 10/6/08 7:46 PM Page 18 FEATURE Nikasil® and Alusil A primer on some of the more common specialty coatings and cylinder materials found in small displacement engines BY JOHN GOODMAN This issue of Engine Professional is Background What Nikasil® and other similar dedicated to specialty engines and niche Any shop considering a cylinder repair coatings are not remains the center of marketing opportunities. Part of that that has been coated with Nikasil® confusion. Alusil’s like Lokasil®, market involves small displacement should be aware of a few material Mercasil® and other high silica substrates engines of all types and configurations. properties and some of the confusion are not coatings at all but rather Past experience with these small bore surrounding them. It will not be the aluminum metal alloys. These alloys have engines has demonstrated a need to purpose of this article to cover many also been referred to as hypereutectic understand materials and coating in a manufacturing details but developing aluminum due to the percent of silica particles contained in the alloy. It is a way quite different from the run-of-the- application and machining do’s and don’ts subtle distinction but Alusil alloys do not mill gasoline engine. Motorcycle, water will be fundamental. contain silica carbide like nickel plating sport, snow mobile, off-road, stationary Let’s begin by covering what Nikasil® does. Instead of silica carbide, Alusil power plant and other specialty engines is. Nikasil® or NiCom® (1967 Mahle employs glass-like silica particles similar offer a machine shop many high profit trademark) is electrodeposited oleophilic to sand. Beside silica particles, other hard opportunities. What has generally kept nickel silicium carbide coating. Simply particle formations are built into the alloy shops from exploiting these niche markets put, it is a nickel plated silica carbide through smelting. These other hard is a lack of knowledge on how to coating often applied to aluminum particles can be regulated in size and approach it, a clear understanding of cylinder bore IDs but can be used for shape depending upon perceived need. what is required by way of equipment many applications where tight tolerances Nikasil® is also not to be confused and an understanding of materials and and superior wear surfaces are indicated. with plasma spray cast iron or carbon coatings used in the manufacture of these The oleophilic feature of Nikasil® gives it coatings. Spray cast iron has been used on engines. a natural tendency to absorb oil, which in aluminum cylinder bore ID’s as a cost- We will attempt to give the reader a turn helps the oil retention of the coating. effective and efficient wear surface. But primer on some of the more common Although there are several proprietary spray cast iron is much softer than coatings and cylinder materials that can formulations of nickel plated silica Nikasil® and subject to greater wear. A be found in these specialty engines as well carbide, application parameters remain quick glance at Nikasil® and spray cast as many automotive engine applications. the same for all of them. iron does not reveal many differences 18 OCT-DEC 2008 engine professional EP 10-2008 18-32 10/6/08 7:46 PM Page 19 Figure 1 Figure 2: Photomicrograph of Electrosil plated cylinder bore because both have a grey look to the for sliding seal components such as silicon levels below 12% are known as surface so further investigation is pistons and rings. hypoeutectic (the silicon is dissolved into necessary to determine what coating you the aluminum matrix). Aluminum with are working with. Carbon coatings are Alusil silicon levels above 12% are known as much easier to identify as they are much Although not a coating, some hypereutec- hypereutectic (aluminum with 16% darker (almost black). Generally, carbon tic aluminum alloys suggest by name that silicon has 12% dissolved silicon and 4% coatings are not found in cylinder bore they possess Nikasil® properties. shows up as primary silicon crystals).). ID’s but have been used successfully in Lokasil®, Mercosil® and Silumal® are A hypereutectic aluminum-silicon that application. but three such alloys and often confused alloy produced by a powder metallurgy with nickel silicon carbide composites. technique comprises 12 to 50% by weight Physical Properties For a more detailed look at this alloy, I of silicon, 1.0 to 5.0% by weight of would refer you to the April-June 2008 copper and 0.01 to 0.05% by weight of Nikasil® issue, pages 20-26 of Engine Professional phosphorus, the content of calcium (Ca) As has been mentioned, many variations magazine and read Tim Meara’s article. as an impurity being controlled to be of Nikasil® like Elnisil®, Kanisil®, Tim is Sunnen Products Senior Honing 0.03% by weight or less. Hypereutectic Electrosil® and SCEM or Suzuki Technician and explains in detail all dis- aluminum-silicon alloy exhibits excellent Composite Electro-chemical Material are creet features of Alusil. It is vital that mechanical strength and is therefore used available from a variety of manufacturers shops do not mix coatings and alloys for manufacturing pistons and other but generally, these nickel silicon carbide together when determining best course of engine components. composites use about 14% silicon carbide action to take with a repair or machining A hypereutectic aluminum-silicon with the rest in nickel and other additives operation. I often hear machinists speak casting alloy used in cylinder blocks for as binders. This would be similar to about preparing a hypereutectic alloy as marine engines is composed by weight of conventional vitrified abrasive material though it were Nikasil®. This can lead to 16% to 19% of silicon, 0.4% to 0.7% where ceramic binders are used to hold disastrous results and ultimately blame magnesium, up to 0.37% copper and the abrasive crystals in place. Figures 1 and 2 the alloy or coating for a failure. So let’s balance in aluminum. With the stated are representations of one type of nickel take a closer look at these hypereutectic silicon content, this alloy has good silicon carbide composite provided by alloys often referred to as Alusil. fluidity and the precipitated silicon Electrosil Performance Cylinder KB Pistons describes hypereutectic crystals provide excellent wear resistance. Technology in Melbourne, Australia. aluminum alloy like this: Silicon additions In addition, the alloy has a narrow By drawing a comparison to vitrified to aluminum are very similar to adding solidification range of less than 150° F., abrasives, one might conclude that sugar to iced tea. Silicon can be added thereby providing the alloy with excellent Nikasil® is an abrasive and by some and dissolved into aluminum so it too castability. The copper content is definitions, it could be. The reason it is becomes inseparable from the aluminum. maintained at a minimum so that the not stems largely from low crystal density If these additions continue, the aluminum alloy has improved resistance to salt and the nickel binder covering all but the will eventually become saturated with water corrosion. microscopic peaks of those crystals. These silicon. Silicon added above this These expressed features of Alusil silicon carbide micro peaks are often saturation point will precipitate out in the have gained popularity as a structural smaller than oil molecules which reside form of hard, primary silicon particles material and less so as a sliding seal wear between crystals and allow pistons and similar to the excess sugar in the iced tea. surface. But many engine manufacturers rings to slide easily along the cylinder This point of saturation in aluminum of all sizes and types use Alusil as a wear surface. I suspect this is one of the reasons is known as eutectic and occurs when the surface too and should be treated Nikasil® exhibits greatly reduced friction silicon level reaches 12%. Aluminum with accordingly. (continued on next page) engine professional OCT-DEC 2008 19 EP 10-2008 18-32 10/6/08 7:46 PM Page 20 NIKASIL® AND ALUSIL BY JOHN GOODMAN Original Equipment Direct Injected Coated Piston for General Motors 2009 2.0L Ecotec Turbo Engine. Machining and Preparation specializes in re-coating with nickel silicon at both ends of an edge and reduce Most of the confusion surrounding carbide. potential separation or flaking. Engine Nikasil® and Alusil exists because they Mahle recommends honing no more manufacturers can do this because they are both used as cylinder ID surfaces for than .001” from Nikasil® coated bores mask off coating just inside the head exactly the same application. Even their and only to restore cylindricity to any gasket sealing ring area of the cylinder. name suggests they are the same so it is wear areas. If wear is greater than .001”, Decking will eliminate this edge feature easy to mix them up and treat them it is advisable to have the coating stripped and may leave the coating unsecured. identically when machining. So how do (in many cases this is a simple electro we machinists and assemblers treat these chemical process and does not require Pistons, Rings and Coatings boring) and fresh nickel silicon carbide Years ago, GM used a hypereutectic alloy two distinctly different processes? applied. Check with the supplier of in their Vega four cylinder engine. That Let’s start with Nikasil®. Nickel coating material for correct honing stones engine had its own problems but silicon carbide composites are applied as a for this process. Please note that piston to rebuilders were left in the dark as to how thin coating of about .002”-.006” thick. wall clearance for Nikasil® coated liners rebuilding should take place.