**Abstract**

For the use of chromium steels in instrumentation, microelectronics, and electrical engineering, their surfaces are additionally protected by coatings based on glass ceramics and other insulating materials. Such materials can operate at high temperatures for a long time under the influence of the electric current or magnetic field. This chapter describes the research results on synthesized coatings based on oxide glass ceramics and oxide materials obtained by plasma chemical vapor deposition (CVD) on the surfaces of stainless steels.

**Keywords:** insulating coatings, glass-ceramic coatings, oxide materials, stainless steel substrate

## **1. Introduction**

It is known that the corrosion-mechanical and functional properties of steels can be improved by alloying elements with a more negative electrode potential (chromium, silicon, aluminum) or elements with a higher potential than iron (e.g., copper, molybdenum, nickel) or elements that promote the formation of carbides, nitrides, and other phases.

The second way (which can occur simultaneously with the first) is to use various methods of surface engineering, in particular, the formation of a dense oxide film, which protects the surface of the alloy from oxidation and dissolution or acts as a functional coating, e.g., a dielectric one. To do this, you need to have the appropriate concentration of the alloying element. To ensure the proper level of functional properties, such coatings must be without defects, with high adhesion to the surface and minimal difference in thermal expansion coefficients as compared to the stainless steel substrate. The condition of formation of such coatings is a certain ratio of the volumes of oxide (VMеО) and the oxidizing metal (VMе): VMеО/VM<sup>е</sup> > 1. Under such a condition, as a rule, coatings are formed that improve the functional properties of the metal and slow down both the degradation process of its microstructure and deterioration of the properties.

As is known, stainless steels are obtained by introducing into the composition of low- and medium-carbon steels at least 12.5% chromium as well as nickel and other alloying elements (titanium, aluminum, molybdenum, niobium, copper, manganese). Depending on the structure, austenitic (AISI 302, 347, 316L, 316, 316Ti, and

others), ferritic (AISI 430, 430Ti, 439, and others), martensitic (AISI 420, 431, 420F), austenitic-martensitic (AISI 631, AM350), and austenitic-ferritic (AISI 301) stainless steels are distinguished. Chromium ferritic and chromium-nickel austenitic steels are the most common in engineering practice [1].

Chromium ferritic stainless steels are the cheapest, but they are inferior to chromium-nickel steels in corrosion resistance. So, AISI 430 steel is stable in acidic environments, but not suitable for welding, because during welding when heated above 900–950°C and rapidly cooled, the grain boundaries become depleted in chromium. With content less than 12% Cr, the electrochemical potential of the steel becomes negative, and it loses its ability to resist corrosion. Under these conditions, AISI 430 steel has a dangerous type of corrosion—intercrystalline corrosion. Stabilization of this steel with titanium or niobium is used to prevent this. It should be noted that σ phase (Fe, Cr) can be formed in high-chromium steels as a result of delamination of the alloyed solid solution into a mixture of α + α<sup>0</sup> phases (where α<sup>0</sup> is σ phase). Due to this, the corrosion resistance of such steels is reduced. In ferritic steels with 20% Cr, the minimum formation time of the σ phase at 600°C is 150 h, and in the steels with 25% Cr at 650°C, it is 15 h. Therefore, when using these steels, it is desirable to bypass this temperature range.

at 1180°C. After holding at this temperature for 60 min, the melt was rapidly cooled to form an amorphous structure and prevent crystallization. The dried granules were ground and sieved to obtain powder fractions with a granule average size of not more than 56 μm. On the basis of the powder obtained, a dielectric paste was made, which was applied to a prepared surface of AISI 420 steel samples and dried at 70°C. For all specimens, standard heat treatment [16–20] was performed with two-step annealing at 380 and 440°C and holding at these temperatures for 45 min. The thermal treatment of the coatings was carried out without a protective atmo-

*Chemical compositions of powders for the synthesis of the PbO-ZnO-B2O3 glass-ceramic system-based*

SC 100-1 75.5 12.0 8.4 2.1 2.0 – SC 90-1 75.3 11.6 8.5 2.1 0.8 1.7 SC 88 75.1 11.2 9.4 1.9 – 1.9 SC 90 75.3 11.6 8.5 2.1 – 2.5

**PbO ZnO B2O3 SiO2 Al2O3 BaO**

The coatings were applied to the surface of AISI 420 stainless steel. To determine

Using the method of interference profilometry, surface profilograms of AISI 420 stainless steel substrates were obtained after various surface treatment methods. From the obtained profilograms using the software "Micron-Gamma," the basic parameters of the roughness of the substrates were calculated, the values of which

The surface of AISI 420 stainless steel with roughness S1 and S3 is characterized by a nonuniform row structure with gaps and projections of the height of 4.6–5.2 μm (**Figure 2**). The surface of AISI 420 stainless steel with roughness S3 after electrolytic etching has a more uniform structure. The height of the needle pro-

Traditional methods do not allow qualitative estimation of quantitative indices of adhesion strength of coatings obtained by the method of thick films, the thickness of which exceeds 100 μm. Because of this, indirect methods of investigation can be used to predict the adhesion strength of such coatings, in particular, the determination of the free surface energy of the substrates on which the coatings are synthesized (**Figure 3**). It is known that the level of surface energy will specify the adhesion properties. This technique allows determining the level of the adhesion

> **Average distance based on the ten highest peaks and lowest valleys over the entire sampling length** *R***Z, μm**

AISI 420—S1 1.11 5.11 4.68 1.02 AISI 420—S2 1.31 5.15 5.36 1.03 AISI 420—S3 1.11 4.00 5.21 1.00

*Surface roughness parameters of the AISI 420 steel substrates after various types of grinding operations [3].*

**Maximum height of the profile** *R***max, μm**

**Average step of peaks along the mean line** *S***m, μm**

the adhesion properties, the steel surfaces were treated with three different methods, which allowed obtaining different indices of surface roughness: S1,

machine grinding; S2, electrolytic etching; S3, manual grinding [3].

**Marking Chemical composition, wt%**

*Technologies of High-Temperature Insulating Coatings on Stainless Steels*

*DOI: http://dx.doi.org/10.5772/intechopen.91334*

sphere to activate the formation of oxides.

jections is uniform and averages 5.3 μm.

**Marking Arithmetical**

**Table 2.**

**59**

**mean deviation of the assessed profile** *Ra***, μm**

are given in **Table 2**.

**Table 1.**

*coatings [2].*
