Mechanical Properties

Introduction

Nickel Silicon-Carbide (NiSiC) coatings are primarily used on the surface of cylinders in order to obtain an increase in wear resistance, better wear patterns and to improve dynamic heat flow as compared to standard cast-iron sleeves or chrome plated cylinders. The coating has also been known to be applied to components which require a lifetime's resistance to wear such as mechanical gearteeth in computer hard-disk drives and the surface of aeroplane hydraulic cylinders. The one use which we are all familiar with is the factory standard NiSiC coating found on nearly all newly purchased engines. Nikadure™ not only compares to the best of the standard factory coatings but it surpasses most of them in terms of longevity.

The Microstructure: NiSiC

The electrolytic coating is made up of a "soft" Nickel matrix which has tiny, hard Silicon-Carbide particles disperesed throughout. The Nickel serves as a 'lubrifying' agent and allows for the moderation of temperature within the cylinder. The Silicon-Carbide particles have a similar function to the skeleton in a human body: they hold the "soft" matrix together. Silicon-Carbide is the third hardest material known to man (after Diamond and Boron-Carbide). The particles used in Nikadure™ are triangular in shape and range approximately from 0.5 microns to 15 microns in size. The concentration and shape of the particles will affect the coating's mechanical performance. During usage, a precipitation hardening mechanism takes place which results in a huge increase in hardness of the coating. The underlying hardening parameters can not be outlined in this document as they are our trade secret. Any original or third-party piston kit can be used. Aurum Plating supplies a wide range of piston kits at an excellent price. Please contact us for details.

SEM Micrograph X1500

As it was mentioned, Aurum Plating undertook a joint study with The University of Queensland in order to better understand the mechanical properties of the coating. This took the form of a thesis (1997-1998) which was supervised by Associate Professor John Drennan of the Microscopy and Microanalysis Centre and Dr Jeff Gates of the Mining, Minerals and Materials Engineering Department. Some of the findings are outlined in the following paragraphs - of course, this content is copyright.

SEM Micrograph X1000

These Scanning Electron micrographs show the microstructure of the Nikadure™ coating. The solid black area represents the Aluminium alloy which makes up the cylinder surface. The lighter grey area shows the Nickel matrix, and the dark triangular shapes are the Silicon-Carbide particles. The following micrograph was taken at X1000 magnification and it displays the density of the SiC particles.

SEM Micrograph X5000

The electroplating process used to produce this microstructure is quite complex. Nickel Plating is achieved by standard electrolisys methodology. The difficult part is being able to adhere the composite ceramic particles (i.e. Silicon-Carbide) to the Nickel matrix. Furthermore, it is desirable to attain an optimal distribution of Silicon-Carbide particles throughout the matrix. This percentage greatly affects the coating's mechanical properties and hence, its performance.

Preciptation Hardening

The hardness of the Nikadure™ coating will increase over time and this is caused by a precipitation hardening process. To speed up this process, the coating can be heated in an oven (for example), but keeping in mind that prolonged exposure to elevated temperatures (above 350° Celsius) could have negative and irreversible effects on the coating's physical properties. The hardening process is widely used in the world of metallurgy. I will not go into too much detail about this hardening process as it is beyond the scope of this web page, but I will however emphasise two things:

1). The mechanics that take place during precipitation hardening result in a "better" microstructure and thus leads to better mechanical performance.

2). Incorrectly carrying out the above process will lead to negative effects - primarily a decrease in hardness. Aurum Plating strongly recommends that all customers who intend to "bake" their barrels, obtain

The Microstructure: Aluminium Alloy (Al380)

The Aluminium Alloy used in the majority of motorbike barrels is commonly known as Aluminiuim Alloy 380 (Al380). It has a reasonably high Silicon content (8.81%) and it serves to enhance the flow of the molten liquid inside the mould's cavities. Other metals including Copper, Iron and Magnesium are also added to improve mechanical properties. As the Al 380 cools down from the molten state, it forms crystals which are called "dendrites". These are simliar in shape to the branches on a pine tree (the microstructure can be seen in the figure below).

Optical microscope

The following 35mm microscope exposure shows the microstructure of the Aluminium alloy used in the barrel. The dendritic formation displayed is proof of crystallographic growth. It can be noted that darker areas situated at the edge of the crystals are actual precipitations of the alloyed metals.