Physical vapor deposition (PVD) coatings are normally about 2-4 microns thick with hardness and chemical stable properties. Tungsten carbide inserts with PVD coatings are well-suited to deal with nickel based, cobalt based or titanium based materials, sometimes even the materials like stainless steel and steel. In addition, in the application of high temperature cutting, the tungsten carbide inserts with PVD coatings also show advanced capability.



In nowadays coating technologies, the most acceptable PVD coatings for the tungsten carbide inserts include titanium carbo-nitride, titanium nitride and titanium aluminum nitride.


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Tungsten carbide AlCrN coated cutting tool is a tungsten carbide cutting tool coated with a thin nano composite film made out of aluminum-chromium-nittride deposited by the PVD process.

Besides the improvement of tungsten carbide cutting tool’s wear behaviour, the AlCrN coating also presents low friction coefficients and wear rates,compared to normal heat treatment coatings. The tungsten carbide AlCrN coated cutting tool is revealed to have a wide potential tribological appliation under the sliding wear condition.



In terms of dealing with the titanium alloy in the machining or aerospace and biomedical components, with adequate process parameters, lubrication and other condition, the tungsten carbide AlCrN coated cutting tool is indicated to display acceptable levels of productivity and better wear resistance as well as better cutting performance due to the coating is mixed with high amounts of aluminum. Generally, the advantages include excellent properties at high temperatures, excellent antioxidation characteristics, as well as anti-spalling and debris removal properties for the contact interface.


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Titanium alloys are widely used in medical and aerospace components manufacturing, which are difficult-to-cut, and the requirements of parallel coated tungsten carbide cutting tools include high performance, enhanced tribological and resistance properties.



Tungsten carbide AlCrN coated cutting tool is a tungsten carbide cutting tool coated with AlCrN coating, which is a thin nano composite film made out of aluminum-chromium-nittride deposited by the PVD process. Since the heat treatment of AlCrN coating allows recrystallization and crystal growth the coated tungsten carbide tool’s wear behavior is enhanced.


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The composition of the hydrosol has not been determined, but it is thought to consist of tungstic acid in combination with water, or possibly with sodium tungstate, since Sabaneeff obtained an amorphous powder, of composition Na₂O.4WO₃, from the dialysed solution.

When the solution is evaporated to dryness, transparent vitreous scales remain, strongly adherent to the crucible; on heating this residue to redness, the trioxide WO₃ results. The aqueous solution has a bitter astringent taste; its density at 19° C. is as follows:

The gelatinisation of silicic acid is retarded by the presence of colloidal tungstic acid.

 

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Colloidal solutions of tungstic acid, in presence of various organic reducing agents such as formaldehyde, sucrose, glucose, dextrin, etc., yield intensely blue solutions on exposure to light. If the solution is kept for some time, it does not undergo this reduction on being exposed to light; but on raising the temperature the blue reduction products are obtained. In order to account for this it has been suggested that two forms of colloidal tungstic acid exist, one being photochemically sensitive and the other not. The former changes spontaneously into the latter, the reverse change being brought about by rise in temperature, and the absorption spectra of the two modifications differ considerably.


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The colloidal solution may also be prepared by dissolving tungsten tetrachloride in alcohol and ether (equal volumes) and then diluting with alcohol and water. The solution obtained acts as a positive colloid and coagulates immediately when small quantities of neutral salts, hydroxides, or strong acids are added. On passing an electric current through the solution, a deep blue precipitate separates at the cathode.

A hydrosol of tungsten hydroxide is readily produced by the electrolysis of a 2 per cent, solution of sodium tungstate between a mercury cathode and a silver anode in a Hildebrand cell. The solution must not be allowed to become acid, or blue compounds are produced. The hydrosols obtained in this way are clear and transparent but brown in colour. The addition of potassium chloride causes coagulation, a black powder, resembling the lower oxides of tungsten, being formed.


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Such formulation suggests a closer relation to the metatungstates than appears to be justified, and would not account for the very essential differences between the two types of compounds.

Paratungstates gradually decompose in aqueous solution with formation of the normal and metatungstates, so that while a freshly prepared solution is neutral to phenolphthalein, it gradually becomes acid on standing - more rapidly on boiling. For this reason the electrical conductivities of the solutions slowly increase at ordinary temperatures.

According to Hallopeau the free paratungstic acid is formed in dilute solution when the barium salt is treated with dilute sulphuric acid. Concentration of the solution, even in vacuo, causes decomposition, and on prolonged boiling, metatungstic acid is formed. Alkalies neutralise the solution, yielding paratungstates. There is, however, no proof that this solution contains any special modification of tungstic acid.


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The content of base to acid in paratungstates was first given by Laurent as 5R₂O:12WO₃, whilst Lotz and Scheibler suggested the formula 3R₂O.7WO₃.xH₂O. Marignac, after careful analysis, concluded that most paratungstates contained 5R₂O:12WO₃, but that a few contained 3R₂O:7WO₃. Other investigators, for reasons mentioned above, were unable to decide between the two formulae. Copaux, from the behaviour of the salts towards dehydration, considered them to be hydrotungstates and gave them the co-ordinative formula R^•₅[H(W₂O₇)₃].aq. In support of this he points to the fact that the paratungstates resemble the complex tungstates in absorbing ultraviolet light, whereas normal tungstates do not do so. Rosenheim suggests that they are 6-tungsto-aquates of composition R₅H₅[H₂(WO₄)₆]aq.


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Solutions of tungstates containing ammonium sulphide yield with hydrochloric acid a brown precipitate of tungsten trisulphide. The addition of zinc chloride to a tungstate solution produces a yellow precipitate which becomes blue on warming with dilute hydrochloric or sulphuric acid. When excess of hydrochloric acid is added to a solution of alkali tungstate and the mixture reduced by means of zinc, brilliant colours, from red to blue, are produced; if phosphoric acid is used, a fine blue precipitate results.

The paratungstates are generally obtained by treating solutions of alkali normal tungstates with acid, or by double decomposition. They can only be obtained from solutions, and always contain water which appears essential to their constitution; it can only be removed with difficulty,.strong heating being necessary for complete dehydration, which is accompanied by decomposition of the salt into the soluble normal salt and the insoluble tetratungstate. From an investigation of the dehydration of the sodium and potassium salts the following results were obtained:
 

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The normal tungstates of the alkali metals are usually obtained by fusing together tungstic anhydride and the alkali hydroxide or carbonate in equivalent proportions. Those of the heavier metals are produced either by double decomposition in solution, or by fusing together an alkali tungstate and the chloride of the metal, often in the presence of sodium chloride. The tungstates of the alkali metals and of magnesium are soluble in water, those of other metals being insoluble, or only slightly soluble, not only in water but also in dilute acids. Concentrated mineral acids (except phosphoric acid) decompose them, with separation of tungstic acid. In this reaction the paratungstates behave similarly, whereas the metatungstates are not decomposed.


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