Influence of Heat Treatment on the Mechanical Properties of Ni Films on 430 Stainless Steel Substrate

Introduction

With the development of metal film preparation technology, the pulse electro-deposition, as a simple, highly active, and low-cost deposition method, had been used to deposit nanocrystalline films [1]. With respect to the deposition of nanocrystalline film, the pulse electrodeposition had been extensively used to improve the hardness, wear, and corrosion resistance of electroplated metal films [1, 2].

Heat treatment was widely used on 304 stainless steel [3], aluminum alloy [4], and nickel films [5] in order to improve their machining properties for eliminating alloy imbalance phase and over saturation [6] as well as reducing surface cracks, and enhancing adhesion strength of film to substrate [7]. Therefore, it is essential to assess the mechanical properties of Ni films on 430 stainless steel substrate at different temperatures, especially their stress–strain relationships. In comparison to traditional methods, nanoindentation is a convenient, nondestructive method, which has been widely used to measure mechanical properties of small samples, such as hardness, Young’s modulus, yield strength, hardening exponent, etc. [8, 9, 10]. The load–indentation depth relationship of an elastic–plastic material was supposed to be a linear combination of elastic and elastic–perfectly plastic response, and then an optimization method was introduced in reverse analysis [11]. In the work of Dao et al. [12], a computational study was undertaken to identify the extent to which the elastoplastic properties of ductile materials could be determined by instrumented sharp indentation and to quantify their sensitivity to variations of indentation data. They concluded that the unique properties can be obtained by a sharp indentation curve. We also proposed an inverse method for extracting the elastic–plastic properties of metallic thin films from instrumented sharp indentation in terms of dimensional analysis and finite element modeling [13, 14]. A wide range of materials with different elastic modulus, yield strength, and strain-hardening exponent were examined.

In this paper, the surface morphology, composition, and texture orientation of Ni films on 430 stainless steel substrate at different temperatures were studied by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The load–indentation depth curves of Ni coatings before and after heat treatment were measured using nanoindentation method. In conjunction with finite element modeling and dimensional analysis, we aim to characterize the stress–strain relationships of Ni films on 430 stainless steel substrate at different temperatures by using a power-law hardening model.

Experimental

Theoretical framework

Material model

For elastic–plastic material, its load–displacement curve is illustrated in Figure 1(a). To improve the accuracy and comparability of hardness and modulus results from nanoindentation experiments, an evaluation of the creep behavior is required. Creep depends on the material and normally diminishes to very low values within some seconds. Nevertheless, it influences the maximum depth and the upper part of the unloading curve in a way that measurement errors of more than 20 % may occur [15]. Hold periods are proposed in dependence on the material type that should be kept for high accuracy measurements. Feng and Ngan solved the problem of indentation on a linear viscoelastic half-space using the correspondence principle between elasticity and linear viscoelasticity [16]. The correction term due to creep in the apparent contact compliance is found to be equal to the ratio of the indenter displacement rate at the end of the load hold to the unloading rate. According to Kick’s law [17], a natural outcome during loading with a Berkovich indenter can be represented as

(1)P=Ch2

Figure 1:

(a) Schematic representation of a load–displacement curve in indentation and (b) the power-law elastic-plastic stress–strain behavior.

where P is the indentation load, h is the indentation depth, and the loading curvature C is a material constant.

As shown in Figure 1(b), for the plastic deformation of metals, a power-law work-hardening law is a fairly good approximation and its corresponding stress–strain curve under uniaxial tension is given by

(2)ε={σE,σ≤σyσyE(σσy)(1/n),σ>σy

It is obvious that there are three parameters, i. e. elastic modulus E, initial yield strength σ_y, and work hardening exponent n. σ_y is the initial yield stress at zero offset strain [18]. Thus, the three parameters (i. e., E, n, and σ_y) are essential to determine the stress–strain curve.

Positive and reverse analyses

As illustrated in Figure 2, there are two steps in either the positive analysis or the reverse analysis [25]. During the positive analysis, finite element calculations are first carried out to obtain several groups of load–displacement curves (defined by F, h, S, and h_c) for given material combinations (defined by E, σ_y, and n). Then, dimensionless functions (П_α and П_β) are determined through numerical fitting to discrete points with proper functions. During the reverse analysis, the load–displacement curve is obtained by nanoindentation tests. Then, П_α and П_β can be calculated by indentation information (i.e. F, h, and h_c). Following the positive and revise analyses [13], σ_y and n can be extracted from the dimensionless functions in

(13)Πα(σyE,n)=(2.77−2.11n)(σyE)−0.064ln(σy/E)−0.42n

(14)πβσyE,n=0.70+0.55exp−2.16n⋅exp−40.32σyE

Figure 2:

Flowchart for extracting the plastic properties of materials from the sharp indentation–loading curve by using positive and reverse analyses.

Because of the scale effect and the bonding strength, it is very difficult to obtain the mechanical properties of thin films directly. Here, we present the reverse analyses method to characterize the stress–strain relationships of Ni films by nanoindentation.

Results and discussion

Figure 3 shows SEM images and EDS analyses of Ni films on 430 stainless steel substrate at different heat treatment temperatures, where the inset is the corresponding SEM images of cross section. For the as-deposited Ni films, a very smooth surface can be obtained with a grain size of 40 nm in Figure 3(a). In the inset, it is clear that the thickness of Ni film is about 3,000 nm. With the increase of heat treatment temperature, the grain becomes bigger, as shown in Figure 3(b) and (c). The grain size increases to ~1 μm for the heat treatment temperature of 800 °C. The EDS analysis of the cross section at 600 °C indicates that the Ni film/Fe substrate system is mainly composed of Ni and Fe species, as shown in Figure 3(d). It is clearly seen that the content of iron is less than Ni in the film though some iron atoms have diffused into the Ni film. Diffusion interface moved because the elements in the film and substrate diffuse to each other during heat treatment, and the interface moves toward the Ni film with a higher diffusion rate, which matches well with the Kirkendall effect [26].

Figure 3:

SEM image of Ni films on 430 stainless steel substrate at different heat treatment temperatures: (a) As-deposited, (b) 500 °C, (c) 700 °C, and (d) EDS of section pattern for Fe and Ni elements at 600 °C.

Texture directions of Ni films on 430 stainless steel substrate at different heat treatment temperatures ranging from 0 °C to 800 °C are examined by XRD in Figure 4. It is found that the structures of the Ni films exhibit a preference of the (1 1 1) and (2 0 0) orientations. With increasing the heat treatment temperatures, the texture directions change slightly, which indicates that no second phase appears. However, the diffraction peaks shift left gradually with increasing heat treatment temperature, implying the formation of new solid solutions.

Figure 4:

XRD patterns of Ni films on 430 stainless steel substrate at different heat treatment temperatures.

Figure 5 shows the load–displacement curves with the maximum load of 1,000 μN for the Ni films at different heat treatment temperatures ranging from 0 °C to 800 °C. It is found that for a given value of 1,000 μN, the maximum penetration depth increases from 60 nm to 110 nm with increasing heat treatment temperatures from0 °C to 800 °C. This indicates a softening effect of Ni films due to the heat treatment process. The indentation creep displacement is about 5 nm, which agrees well with previous results [27, 28, 29]. For consistency, only load–displacement profiles that mapped onto the same master curve were admitted as valid measurements (the invalid profiles have a fraction less than 5 % for each film we tested).

Figure 5:

The load–indentation depth curves of Ni films on 430 stainless steel substrate at different heat treatment temperatures.

The influence of the heat treatment temperatures on the measured elastic modulus and hardness values for Ni films is shown in Figure 6(a) and (b), respectively. Both, the elastic modulus and hardness show a strong heat treatment temperature effect with the apparent elastic modulus and hardness decreasing with increasing heat treatment temperatures. For low heat treatment temperatures, the elastic modules are about 160 GPa which is insignificant to the temperatures. However, the heat treatment temperature increases to 800 °C, the elastic modulus value decreases to 70 GPa suddenly, as shown in Figure 6(a). In addition, the hardness value decreases from 7.5 GPa to 3 GPa with the increasing heat treatment temperatures from 0 °C to 800 °C in Figure 6(b). This can be attributed to the increasing grain size due to heat treatment. The smaller the grain size, the stronger the strength, which agrees well with Hall–Petch effect [30]. Furthermore, according to the investigations by Buchheit et al. [31], the evolution of microstructure in the annealed Ni materials corresponded with a dramatic drop in their strength and determined the limiting diffusion-bonding temperature. Significant strength decreases occurred during heat treatments at temperatures greater than 400 °C, corresponding to recrystallization and rapid grain growth of the recrystallized microstructure.

Figure 6:

Temperature dependence of (a) elastic modulus and (b) hardness.

According to the reverse analysis, the mechanical properties (E, σ_y, and n) can be derived from the measured load–displacement curves and their corresponding data (see Table 1). Then, the stress–strain curves are described at different heat treatment temperatures ranging from 0 °C to 800 °C. As shown in Figure 7, the yield strength and elastic modulus decrease with the increase of heat treatment temperature. From Table 1, it can be seen that the elastic modulus decreases from 148 GPa (0 °C) to 69 GPa (800 °C). There is a similar tendency for the decrease of yield strength with the increase of heat treatment temperature. In the case of 0 °C, yield strength is 1.79 GPa. With the increase of heat treatment temperature to 800 °C, yield strength gradually decreases to ~400 MPa. In addition, it is worth noting that, as shown in Table 1, there is a slight change of the n values around 0.4 with the increase of heat treatment temperature.

Figure 7:

The stress–strain relationships of Ni films on 430 stainless steel substrate at different temperatures.

Conclusion

In this paper, the Ni films on 430 stainless steel substrate systems are prepared by pulse electrodeposition at different heat treated temperatures ranging from 0 °C to 800 °C. Using a combination of dimensional analysis and finite element modeling, we extracted successfully the elastic–plastic properties of Ni films at different heat treatment temperatures by an inverse method. The results shown that, with the increase of heat treatment temperatures, the elastic modulus and yield strength obviously decrease. It is expected that this study can be used in the characterization of mechanical properties for metallic films at the heat treatment processes.