Metallo-organic deposition of tungsten oxide films from alkylammonium tungstate solutions

This paper describes a simple and inexpensive metallo-organic deposition (MOD) process for forming electrochromic tungsten oxide (WO₃) films on glass. The thin films of WO₃ were made by air firing (500–700°C) films from xylene/2-propanol solutions of bis-(di-n-octylammonium) tetratungstate, [(n-C₈H₁₇)₂NH₂]₂[W₄O₁₃]. The process coats glass with undoped films ranging in colour from faint yellow to dark brown, and can be used to make gradients of these colours. The colour is determined by the firing parameters and results from residual carbon and tungsten suboxides in the film due to incomplete firing. Increased firing temperatures or longer firing times removes the carbon and produces films with higher crystallinity. Electrochemical doping with acid (H⁺) switches the colour gradient films to a uniformly blue colour.

When the temperature rises from 1150 ℃ to 1250 ℃, tungsten skeleton density from the 12.15g / cm3 increased to 12.39g / cm3. However, after that sintering temperature continues to rise, in the third stage and post-sintered tungsten skeleton, the density change is small. At this moment, the majority of the pores are completely separated, closed cell number is greatly increased, and pore shape tends to be spherical and shrinking, is not conducive to the process of post-infiltration.

From the aspects of sintering kinetics, the sintered tungsten copper alloy electrodes tungsten powder mainly diffusion and flow of two different mechanisms. Especially in the media and high temperature sintering, surface diffusion becomes very obvious. At this time due to the fine-grained powder, which powder is larger than the surface area of the active surface of the large number of atoms, the diffusion surface it is relatively easy to carry out, which also makes sintered tungsten skeleton easily enter the third stage from the second stage.

More information about sintering process effect on tungsten copper electrode properties, click here:

http://news.chinatungsten.com/en/tungsten-information/80415-ti-10319
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Tungsten powder unit-based sintering can be divided into three sections, which include coupling phase, sintered neck formation, closed pores spheroidization and reducing phase. In order to obtain the desired tungsten skeleton, we should try to control the sintering process within the second section, which is beneficial for porosity reducing and ensuring the best powder sintering temperature of different granularity.

As the temperature increases, the density of tungsten copper electrodes increase in the first two stages of sintering. Initial contact point or contact surface of tungsten powder changes into crystalline combination, through nucleation, growth and other atomic processes to form a sintered neck, then to a large number of atoms migrate bonding surface tungsten particles makes the sintering neck to expand, narrow particle spacing and form continuous pore network, and because of grain growth, grain boundary migration began, the area swept by grain boundary pores will miss, thereby forming tungsten skeleton connection.

More information about sintering process effect on tungsten copper electrode properties, click here:

http://news.chinatungsten.com/en/tungsten-information/80415-ti-10319
http://news.chinatungsten.com/en/tungsten-information/80416-ti-10320

In last part we know that the (Al,Cr)N coating has lower oxidation temperature and a lower coefficient of friction, which also explains less wear for (Al,Cr)N coated tungsten carbide tool. And this part we make experiments to test the influence of cutting speed on tool wear.

As mentioned above, (Al,Cr)N was found to have superior properties for coating material  compared to (Ti,Al)N in cutting hardened sintered steel.

In cutting hardened sintered steel at various cutting speeds, the wear progress of tungsten carbide tool was investigated at a feed rate of 0.05 mm/rev and a depth of cut of 0.1 mm. Fig. 4 shows the influence of the cutting speed on the tool wear. There is little influence of the cutting speed on the tool wear within the range of cutting speed from 0.5 m/s to 1.0 m/s. So, SEM observation and EDS analysis was carried out on the worn surface.

(To be continued. This article is divided into several parts. Here is part 9. For part 8 please refer to http://news.chinatungsten.com/en/tungsten-information/80629-ti-10438)

In last part we know that the (Al,Cr)N coating is harder than the (Ti,Al)N one and make the coated surface of tungsten carbide tool more resistant to abrasive wear which is the main wear mechanism in our experiments. And this part we will explain the second reason of why the wear progress is slower for (Al,Cr)N coated tungsten carbide tool.

Second, as compared with the oxygen element on the worn surface of the (Ti,Al)N coating film shown in Fig. 3 i(b) and that of the (Al,Cr)N coating film shown in Fig. 3 ii(b), the oxygen element of the (Al,Cr)N coating film, which was turned at long cutting distance, is  less than that of the (Ti,Al)N coating film. Therefore, the cutting temperature of the (Al,Cr)N coating film is considered to be lower than that of the (Ti,Al)N coating film, and the wear progress of the (Al,Cr)N is slower than that of the (Ti,Al)N. One reason for the lower cutting temperature of the (Al,Cr)N coating film is as follows.

That is, the (Al,Cr)N coating film has a lower coefficient of friction as compared with the (Ti,Al)N coating film. Furthermore, the (Al,Cr)N coating film has a higher oxidation temperature as shown in Table III. This is considered to be the reason why there is less tool wear of the (Al,Cr)N coated tungsten carbide tool.

(To be continued. This article is divided into several parts. Here is part 8 For part 7 please refer to http://news.chinatungsten.com/en/tungsten-information/80628-ti-10437; for part 9 please refer to http://news.chinatungsten.com/en/tungsten-information/80632-ti-10439)

In last part we know that as the main tool failure mode, flank wear of (Al,Cr)N coated tungsten carbide tool is slower than the (Ti,Al)N coated one. And this part we will get to understand of the wear mechanism of the two coating films on tungsten carbide tools.

Fig. 3 shows the SEM observation and EDS mapping analysis on the worn surface of the (Ti,Al)N coated tungsten carbide tool and the (Al, Cr)N coated tool. Fig.3 (a) shows the details of “A”shown in Fig. 1. Fig. 3 (b) shows the EDS analysis in the case of the oxygen (O) mapping on the cutting part shown in Fig. 1.

First, as compared with the worn surface of (Ti,Al)N coating film indicated by “B” shown in Fig. 3 i(a) and that of(Al,Cr)N coating film indicated by “B” shown in Fig. 3 ii(a), many striae scratched by a hard material are remarkably found on the worn surface in the case of both the (Ti,Al)N and the (Al,Cr)N coating film. Therefore, the main wear mechanism of both the (Ti,Al)N and the (Al,Cr)N coating film is considered to be abrasive wear. In the case of abrasive wear, the wear-resistance of the coating film often depends on the hardness of the coating film. That is, a coating film with higher hardness has good wear-resistance. So the characteristics of the coating films were investigated. The results are shown in Table III. The thickness of coating of (Al,Cr)N film is 3 μm, which is the same as that of (Ti,Al)N film. The (Al,Cr)N coating is harder than the (Ti,Al)N coating because the wear progress of (Al,Cr)N is considered to be slower than that of (Ti,Al)N.

(To be continued. This article is divided into several parts. Here is part 7 For part 6 please refer to http://news.chinatungsten.com/en/tungsten-information/80627-ti-10436; for part 8 please refer to http://news.chinatungsten.com/en/tungsten-information/80629-ti-10438)

In last part we know that after testing, tungsten carbide tools with (Al,Cr)N coating film exhibit much better wear resistance quality than with (Ti,Al)N coating. In this part we will get to a closer analysis of the flank wear feature of the two coated tungsten carbide tools.

Following these experiments, it emerged that the main tool failure for two kinds of coated tools in cutting hardened sintered steel was the flank wear within a maximum value of the flank wear width of about 0.2 mm. Therefore, the maximum value of the flank wear width (VBmax) was measured by microscope.

Fig. 2 shows the wear progress in cutting two kinds of coated tools at a cutting speed of 1.00 m/s. The wear progress of the (Al,Cr)N coated tool is slower than that of the (Ti,Al)N coated tool. As mentioned above, the (Al,Cr)N coated cemented tungsten carbide tool is an effective tool material in cutting hardened sintered steel, so Scanning Electron Microscopy (SEM) observation and Energy Dispersive X-ray Spectrometer (EDS) analysis were carried out on the worn surface.

(To be continued. This article is divided into several parts. Here is part 6. For part 5, please refer to http://news.chinatungsten.com/en/tungsten-information/80617-ti-10434; for part 7, please refer to http://news.chinatungsten.com/en/tungsten-information/80628-ti-10437)

In last part we know that the tungsten carbide tools PVD-coated with (Ti,Al)N and (Al,Cr)N coating films are tested in cutting hardened sintered steel to compare the effect, since (Al,Cr)N coating film is a new film. In this part we will get result and make analysis.

Results and Discussion

The hardened sintered steel was turned with two kinds of coated cemented tungsten carbide tool at a feed rate of 0.05 mm/rev and a depth of cut of 0.1 mm. Fig.1 shows the SEM observation of the tool wear. Fig.1 (a) and (b) show the case of the (Ti,Al)N coated and the  (Al,Cr)N coated tool at a cutting speed of 1.00 m/s, respectively. In the case of the (Ti,Al)N coated tool shown in Fig. 1 (a), a crater is found on the rake face along the cutting edge, and adhesion on both the rake and flank faces is not remarkable either. Also, the flaking of the coating layer is not remarkable. In the case of the (Al,Cr)N coated tool shown in Fig. 1 (b), there is no remarkable crater on the rake face. And adhesion on the flank face is found, while the flaking of the coating layer is also unremarkable.

(To be continued. This article is divided into several parts. Here is part 5. For part 4, please refer to http://news.chinatungsten.com/en/tungsten-information/80616-ti-10433)

In last part we know that the study is to explore the effectiveness and wear mechanism of tungsten carbide tool with (Al,Cr)N coating film coated by PVD technology in turning hardened sintered steel.This part goes on explaining how the experiments go.

2. Experiment Details

The work material used was hardened sintered steel from diffusion alloyed powder. This material had a density of 7.1 Mg/m3, and many pores were observed when examining the structure by microscope. The chemical composition and the mechanical properties of the hardened sintered steel are shown in Table I. That is, the hardened sintered steel had a hardness of 339 HBS. The cutting conditions are shown in Table II. The tool material of the substrate was cemented carbide, and two types of PVD-coated cemented tungsten carbide tool were used. Namely, the coating films used were (Ti,Al)N and (Al,Cr)N coating film. (Al,Cr)N is a new type of coating film. The configurations of tungsten carbide tool inserts were TNGA160408. The insert was attached on the tool holder MTGNR2525M16. In this case, the tool geometry was (-6, -6, 6, 6, 30, 0, 0.8 mm). The turning tests were carried out on a precision lathe (Type ST5) with an added variable-speed drive. The driving power of this lathe is 7.5/11kW and the maximum rotational speed is 2500 min-1. The tool wear of the two kinds of turning insert was investigated.

(To be continued. This article is divided into several parts. Here is part 4. For part 3, please refer to http://news.chinatungsten.com/en/tungsten-information/80615-ti-10432; for part 5, please refer to http://news.chinatungsten.com/en/tungsten-information/80617-ti-10434)