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Boron Nickel. Autocatalytic technology for new applications and as a suitable replacement for hard chrome.

Article by J R House (Surface Engineering Consulting Ltd and UCTI ) and E McComas (Universal Chemical Technologies Inc.)

Autocatalytic technology is well known particularly in the Phosphorus / Nickel form. An alternative, for high build applications, has been to use Boron / Nickel, but until relatively recently the systems were chemically unstable and considered to be expensive. This paper looks at the properties of the deposit and the applications that make use of them and looks at the technology as a potential replacement for hard chrome.

INTRODUCTION

Dupont introduced electroless Boron Nickel to the UK in the late 70’s and early 80’s. While employed with IMASA Ltd, we installed a system (Sylek 103) in Yorkshire trying to take advantage of the high melting point and the ability of the deposit’s self releasing properties. We lined steel moulds which ultimately were used for blowing glass Coca Cola bottles. The technology worked well from the moulder’s point of view. Hence there was nothing wrong with the claims made for the deposit.

Boron / Nickel was also seen as a replacement for hard chrome, in days when pollution prevention of the River Aire wasn’t given the same attention as it is today. In addition, comparison costs generally only considered the plating costs and so few accounted for the post machining of chrome or effluent treatment costs of a chrome solution. Had these factors been taken into account, then the cost difference wasn’t as large as imagined.

From a plating perspective, there were problems. The bath was either unstable or over-stable. While supply house chemists could operate the bath, regrettably the window of operation was far too narrow for the system to be controlled correctly in a typical plating environment. Hence the whole technology went onto the back burner until an enterprising Ed McComas bought the Patents from Dupont and started to look at the chemistry and subsequently the properties of the deposit.

The Nature of the Boron/Nickel Process

The boron/nickel (hereafter BNi) high build deposit has always had around 5 to 6% Boron and is plated from a highly alkaline bath (pH ~ 12.5). To keep the nickel in solution one requires a strong complexant and this is Ethylene Diamine (EDA) and so the solution is a deep purple colour. Don’t let that put you off. EDA is much easier to handle than its close cousin EDTA, but more of this later. In addition we need to add stabilisers, which prevent random seeding out and allow the reduction reaction to occur on the catalytic surface. Finally the reducer is Sodium Borohydride. The whole mixture operates at around 90 to 92°C and will have a plating rate of around 20 microns per hour.

Control is relatively simple. The nickel content can be determined spectrophotometrically. A simple acid base titration, using an electrometric endpoint detector (Metrohm autotitrator) will show four endpoints. The first relates to the free caustic (NaOH), the next two relate to the free EDA and the fourth relates to the EDA complexing the nickel. From the latter one can also calculate the complexed nickel and correlate that with the earlier determination. However for most purposes a regular check on the nickel content and appropriate replenishment keeps the bath in control. This is a huge improvement on the original system. The stabiliser system developed by Ed McComas is patented.

The Nature of the Deposit Structure

The BNi deposit is intended for engineering applications. It is a matt, grey colour with little aesthetic appeal. The structure is dendritic and therefore it grows in columns from the substrate. Consequently the surface structure is nodular. Hence while the BNi deposit is corrosion resistant, the structure does not lend itself to corrosion protection. If corrosion protection is important, coupled with some property peculiar to the BNi deposit, then an undercoat of conventional electroless phosphorus nickel must be applied.

Nodular growth with true metallurgical


It is also true to say that if the BNi deposit is operating in an oil environment (cylinders, pistons, etc.) the structure absorbs the oil by capillary action and therefore the surface never runs dry even if most of the oil has departed to the sump. At the same time the oil provides the corrosion protection.

Lubricity

An outcome of the structure is that where the BNi surface meets another only 25 to 30% of the surface will be in contact. This reduces the frictional drag. This is very noticeable when two BNi surfaces rub together. The result is that the surface has a low coefficient of friction. This property lends itself to applications where a lubricious surface could be advantageous.

NOTE:- UltraCem is the trademark of UCTI for the BNi deposit.

Hardness

It is impossible to determine the hardness of an electroless BNi deposit by creating a diamond impression of the surface. The diamond is just as likely to hit the spaces between the columnar growth as to hit the columns. The only acceptable way for hardness determination is to section a minimum 25micronsdeposit and create an indent into the side of the section. Under these circumstances Knoop Hardness figures are around 1300 to 1400 HK25

This now begs the question whether the BNi deposit is a suitable alternative to hard chrome? Clearly the hardness coupled to the uniformity of coating at least makes such deposits a potential contender as a hard chrome replacement.

Wear Characteristics

Medium phosphorous electroless nickel has good wear characteristics, and is used on items such as cogs and gears. Is BNi also wear resistant?

An alternative way of looking at the wear characteristics is given below.

The Falex Corporation, an independent laboratory specialising in wear testing and test equipment, performed the above test.

...continued (Click here)

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