JULY 1984 ANTI-CORROSION 9 (iii) The remelting of the coating may significantly affect In last month's "Corrosion Commentary" we gave brief the properties of the substrate; i.e. grain size. In addition, details of the new Hy-Pac process from Plasma Coatings the creation of a localised liquid film promotes high surface Ltd. to produce non-porous thermally sprayed coatings. We stress during cooling, and in complex shapes there is a danger have now received the following further information. of producing unequal coating thickness, due to gravity and Plasma Coatings report that thermally sprayed coatings surface tension effects and, in addition, possible distortion produced by a variety of techniques contain inherent porosity of the component. typified by the particular process employed. As porosity can often degrade the performance of a coating, it is desirable 4. During an extensive R& D programme Plasma that wherever possible the volume fraction porosity should Coatings has devised a technique of coating deposition in be minimised. which the coating has a laminar structure, removed of porosity and particle boundaries which is metallurgically bonded to the substrate. Current techniques The company has made application for patent protection Several routes have been developed for the production of for this technique. dense coatings but, to a greater or lesser degree, all exhibit The following examples illustrate the advantages afforded the disadvantages referred to above. by the new technique:— 1. Welded deposits: which generally require excessive amounts of material to achieve final machining to desired limits. They often require recycled processing. Titaniu m carbide/cobalt 2. (a) By oxy-acetylene sprayed powder or wire deposits Can be applied by the plasma process as a high temperature which give medium to high porosity level and, more impor anti-fretting material. tantly, relatively weak adherence to the substrate. Such a coating normally fails due to growth of oxides at (b) By high velocity plasma gun it is possible to restrict the coating/substrate interface resulting in coating detach porosity to low levels. Isolated porosity is unfortunately still ment. present, but adherence of the coating to the substrate is Since the coating structure and its attachment to the considerably better than in 2(a). substrate is of a mechanical nature, the expansion/contrac tion effects of temperature cycling promote interparticle and (c) Wear resistant coatings deposited by the oxy-acetylene interface cavitation which allows oxygen to permeate through detonation process which, though relatively expensive, the coating. achieves higher bond strengths than in (a) or (b). The absence of porosity/particle boundaries and promo tion of diffusion bond resulting from the "Hy-Pac" tech 3. A specially formulated coating may be applied, nique enhances coating life. employing methods 2 (a) or 2 (b) which, in a second opera tion is remelted or "fused" to the substrate. Such coatings are known as fused coatings. However, the reversion to a liquid phase during fusing promotes gas entrapment and Copper alloys and copper/silicon carbide shrinkage porosity. Diffusion effects at the coating/substrate A disc or drum brake surface can be coated with a copper interface are rapid, which develops good bonding. Neverthe based material containing second phase particles, which less, the fusion reaction has a number of major disadvan combines good heat transfer properties with excellent wear tages. resistance. (i) Coating and substrate are often depleted in elements By conventional methods, a layer of nickel aluminide or which are important to their ultimate function—i.e. streng molybdenum is applied to the substrate in advance of the thening agents in the substrate are removed to the coating, coating to enhance bonding. Unfortunately, such a layer thereby reducing the strength of the substrate at the inter reduces thermal conductivity and thereby adversely affects face. Conversely, the removal of, for example, chromium braking efficiency. from the coating into the substrate reduces the oxidation Employing the Hy-Pac technique, the copper based wear characteristics of the coating. resistant alloy may be applied to the substrate in such a way (ii) The conversion of the coating to a liquid film pro that the metallurgical bond at the interface and absence of motes the rapid solution of any second phase particles within particle boundaries ensures maximum heat dissipation. The the coating, e.g. tungsten carbide. system also obviates the need for a sealer. Such particles are added to enhance wear resistance of the coating and, as such, this property is reduced by con A similar technique can be applied to aluminium alloy ventional fusing. Second phase particles remaining have a substrates, thus effecting a reduction in weight of the brake tendency to move by gravitational or flotation effects, component. thereby reducing the uniformity of hard particles within • Plasma Coatings Ltd., 5, Meverill Road, Tideswell, North the matrix. Derbyshire SK17 8PY.
Anti-Corrosion Methods and Materials – Emerald Publishing
Published: Jul 1, 1984
It’s your single place to instantly
discover and read the research
that matters to you.
Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.
All for just $49/month
Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly
Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.
Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.
Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.
All the latest content is available, no embargo periods.
“Hi guys, I cannot tell you how much I love this resource. Incredible. I really believe you've hit the nail on the head with this site in regards to solving the research-purchase issue.”Daniel C.
“Whoa! It’s like Spotify but for academic articles.”@Phil_Robichaud
“I must say, @deepdyve is a fabulous solution to the independent researcher's problem of #access to #information.”@deepthiw
“My last article couldn't be possible without the platform @deepdyve that makes journal papers cheaper.”@JoseServera