Repetitive Nanosecond Laser-Induced Oxidation and Phase Transformation in NiTi Alloy

A commercial equiatomic NiTi alloy was utilized, and it was mechanically polished with 100, 600, 1000, and 2000 grit SiC paper to remove contaminants and create a smooth surface for laser treatment. The alloy's surface was irradiated using a nanosecond ytterbium fiber laser (IPG Photonics, USA), capable of providing a peak power of 15 kW, a pulse duration ranging from 4 to 200 ns, and repetition rates from 2 to 1000 kHz. The NiTi alloy surfaces were exposed to laser light with a wavelength of 1064 nm, an output power of 4 W, a pulse duration of 4 ns, a scanning speed of 0.5 m/s, and a repetition rate of 800 kHz, as illustrated in Fig. 1. The polished samples underwent irradiation in a single direction, with the laser beam being precisely controlled in terms of interval gap and overlap frequency. The laser irradiation width was 32 μm, and the interval gap between irradiating beams was 12.8 μm, resulting in a 60% overlap. Subsequently, depending on the surface roughness, we will refer to the sample after a single irradiation (n = 1) as 'nanopattern' and the one after 16 irradiations (n = 16) as 'micropattern' based on the surface roughness.

The irradiation-induced surface roughness of the samples was evaluated by capturing scanning electron microscopy (SEM) images at a 52° tilt angle and by obtaining cross-sectional images after ion milling using a dual-beam focused ion beam (FIB) instrument (Helios NanoLab 600, FEI). The roughness of the patterned surface was quantified and characterized by atomic force microscopy (AFM) (XE-100, Park System). The non-contact mode was used to inspect the surface using a non-contact cantilever (PPP-NCHR, Park System). To analyze the multi-layered cross-sectional structure generated through laser beam irradiation, cross-sectioned TEM lamella samples were prepared using FIB. For the surface protection, Pt was deposited on the samples. The phase evolution beneath the surface was characterized in terms of crystal structure using selected area diffraction patterns (SADPs) and in terms of chemical composition through energy dispersive spectroscopy (EDS) analysis in a TEM (Talos F200X, FEI) equipped with EDS having four SDD detectors.

The pristine NiTi alloy exhibits no distinctive features aside from surface polishing scratches, as seen in Fig. 2a. The arithmetical mean roughness, \({R}_{a}\) measured by AFM is 9.8 nm. Following a single laser irradiation (n = 1), a faint wavy pattern, referred to as ‘nanopattern’ emerges on the surface, with the \({R}_{a}\) of 27.2 nm, as more clearly depicted in the tilted image in Fig. 2b. After 16 rounds of laser irradiation, the surface roughness intensifies, yielding an \({R}_{a}\) of 732.5 nm, termed the ‘micropattern’ (Fig. 2c). The scale of the surface roughness, contingent on laser irradiation parameters, can be classified as macro-, micro-, nanometer-sized textures, each exhibiting varying effects on implantation. It is well-known that the micrometer-scale roughness enhances implant fixation to bones, while the nanometer-scale roughness relates to protein adsorption and the adhesion of osteoblastic cells [38]. Cross-sectional SE images are used to roughly measure oxide thickness and the heat-affected zone (HAZ). In the nanopattern sample, the HAZ measured as 218 nm, including the approximately 59 nm oxide film on the surface(Fig. 2b). Conversely, the HAZ depth significantly increases to 4.3 μm with 656 nm oxide film in the micropattern sample (Fig. 2c).

To assess the oxide structures, TEM analysis was conducted on three samples: the pristine sample, the nanopattern sample, and the micropattern sample. The pristine sample was divided into four layers with contrasting features along the depth axis, as depicted in the bright-field (BF) TEM image in Fig. 3a. The predominant phase within the matrix is B19′ phase [39‐41], which remains thermodynamically stable at room temperature due to its high martensite start temperature (\({T}_{Ms}\)) inherent in equatomic NiTi alloy (as shown in Fig. 3a) [42, 43]. Above the matrix, a 24 nm-thick Ni-rich amorphous layer (Ni: ~ 59 at%, Ti: ~ 41 at%) was observed.

At the top surface, there exists a continuous 4 nm-thick layer displaying a distinct bright contrast. Chemical composition analysis via EDS revealed a composition close to TiO₂ (Ti: ~ 28 at%, O: ~ 71 at%). This continuous amorphous Ti oxide layer, often referred as the native oxide, is a characteristic feature found on NiTi alloys and Ti-based alloys at room temperature, primarily due to the selective oxidation of Ti, which has a strong affinity [34] for oxygen. In the literature [44], it has been reported that the thickness of native oxide on Ti metal at room temperature typically falls within the range of 2–7 nm, consistent with the findings of this study. The selective oxidation of Ti on the surface induces the inward diffusion of Ni toward the interface between the oxide and the matrix, resulting in the formatin of a Ti-depleted Ni–Ti amorphous layer (Ni: ~ 69 at%, Ti: ~ 31 at%) with a thickness of approximately 10 nm beneath the Ti oxide layer. Below this, there exists a Ni₃Ti₂ amorphous layer (Ni: ~ 59 at%, Ti: ~ 41 at%) with a thickness of approximately 24 nm. All three of these layers present on the surface possess amorphous structures which is confirmed through the halo ring pattern observed in the fast Fourier transform (FFT) in Fig. 3a. High-angle annular dark-field (HAADF) STEM imaging and corresponding EDS mapping, as shown in Fig. 3b, c, clearly visualizes the Ti oxide layer along with the underlying Ni-enriched layers on the surface.

Following a single laser irradiation, the surface structure undergoes a significant transformation, as illustrated in Fig. 4, resulting in the division of the HAZ into three distinct layers with a depth of 340 nm. At the uppermost surface, a Ti oxide layer exhibits a bright contrast, extending to a thickness of approximately 66 nm, a substrantial increase compared to the 4 nm native oxide observed in the pristine sample. Notably, spherical Ni particles ranging in size from 2 to 17 nm, displaying a gray contrast, are embedded within the Ti oxide film. The FFT pattern obtained from the oxide layer, including the embedded Ni particles, indicates the crystalline nature of the Ni particles, while the Ti oxide maintains its amorphous structure. Under atmospheric thermal oxidation conditions, Ni cations do not typically form Ni particles within the Ti oxide due to the selective oxidation of the Ti element; instead, they accumulate beneath the Ti oxide film[35, 45], as observed in the pristine sample (Fig. 3). However, the rapid solidification induced by nanosecond laser irradiation may facilitate the formation of some embedded Ni particles, as it does not provide sufficient time for Ni to diffuse below the oxide film. The HAADF STEM image and the corresponding EDS elemental map in Fig. 4b clearly depict the presence of trapped Ni particles within the Ti oxide layer.

Below the Ti oxide layer, a featureless dark contrast layer with a thickness of approximately 163 nm has been identified as amorphous NiTi, exhibiting a chemical composition close to Ni₃Ti₂. The relatively high Ni content (62 at%) comparison to the Ni₅₀Ti₅₀ matrix can be attributed to partial inner diffusion of Ni due to Ti oxide layer growing during laser irradiation. Furthermore, Ni₃Ti₂ alloy demonstrates sufficient glass-forming ability [46] to maintain its amorphous structure even after undergoing melting and re-solidification processes through rapid quenching induced by laser irradiation.

The third layer, not distinctly discernible from the B19’ phase of the matrix due to similar chemical composition and crystalline characteristics in the SEM image (Fig. 2c), has been identified as the B2 NiTi phase through SADP analysis. This phase transformation from the B19’ to the B2 phase can be attributed to the dependency of the martensitic transformation (MT) temperature on chemical composition. The EDS line profile analysis in Fig. 4a reveals a continuous variation in Ni and Ti content within the B2 NiTi layer. As a consequence of the outward diffusion of Ti, driven by selective oxidation towards the interface of amorphous Ni₃Ti₂ and B2 NiTi during surface melting induced by laser irradiation, the Ni concentration in the B2 NiTi layer becomes slightly elevated compared to the B19’ matrix. The MT temperature of binary NiTi alloys is highly sensitive to Ni content, as illustrated in Fig. 4c, where a 1 at% increase in Ni content results in a reduction of the austenite finish temperature (\({T}_{Af}\)) and martensite start temperature (\({T}_{Ms}\)) by more than 100 K [47‐49]. Consequently, when the Ni content surpasses 50.7 at%, the B2 austenite phase becomes more stable compared to the B19’ phase at room temperature due to the lowered \({T}_{Af}\) [42]. In other words, the decrease in \({T}_{Af}\) within the Ni-rich region of the matrix triggers the formation of the B2 layer above the B19’ matrix. As a result, the surface of the nanopatterned sample comprises four layers: crystalline B19’ and B2 NiTi phases, along with amorphous Ni₃Ti₂ and Ti oxide embedded with Ni nanoparticles.

Repeated laser irradiation, conducted 16 times (Micropattern), leads to the formation of a thicker oxide layer, approximately 695 nm in thickness, comprising multiple oxide layers (Fig. 5a). Notably, a continuous NiO layer with a thickness of around 10 nm is confirmed above the rutile phase thorugh EDS elemental mapping (as depicted in Fig. 5b). Building upon the presence of trapped Ni particles within the Ti oxide following a single laser shot, as observed in Fig. 4, we hypothesize that these trapped Ni particles act as a source for supplying the final thin NiO layer on the top surface after repeated laser irradiations.

Another intriguing observation in the samples subjected to repeated laser irradiation is the presence of not just a single Ti oxide layer, but three distinct oxide layers, namely rutile, anatase and fcc TiO layers. It is hypothesized that these oxide layers are formed through a three-step process: i) the intial formation of the anatase phase, ii) simultaneous out-diffusion of excess Ni elements from the anatase phase, leading to the formation of an amorphous Ni oxide layer, and iii) a phase transition from anatase to rutile.

Let us first consider the appearance of anatase. Fathi-Hafshejani et al. demonstrated that the surface oxide structure of Ti or Ti alloys is influenced by the heat generated during laser processing, including factors such as laser power, scanning speed, laser pulse duration, and frequency [50]. Under conditions of higher laser energy, frequence and slower irradiation speed, rutile TiO₂ tend to preferentially form, whereas lower laser power and/or faster irradiation speed promote anatase formation. It is worth noting that the laser irradiation conditions employed in our study, specifically a laser power of 4 W and an irradiation speed of 0.5 m/s, can be categorized as relatively mild conditions, potentially falling within the unaffected zone of the heatmap presented in reference [50]. However, the higher frequency (800 kHz) and repeated laser scans, which provide sufficient power density, may contribute to the formation of the anatase phase.

Next, let’s examine the structure of the rutile phase. The formation of the rutile phase typically demands a higher amount of thermal energy compared to the anatase phase [50]. However, our experiments do not align with this scenario. Instead, it is anticipated that the excess Ni atoms in anatase TiO₂, which remains even after the repeated laser scans, trigger a phase transformation from anatase to rutile [51‐53]. These excess Ni atoms substitute into the Ti sublattice of the anatase phase, creating oxygen vacancies due to the difference in valence between Ti⁴⁺ and Ni³⁺ ions. These oxygen vacancies induce relaxation within the large oxygen sublattice, leading to atomic rearrangement and the transformation from anatase to rutile [51]. The detailed mechanism for this phase transformation is depicted in Fig. 5c.

Lastly, let us consider how the TiO phase forms beneath anatase TiO₂. TiO₂ can undergo conversion into the TiO phase when there is insufficient oxygen supply. The presence of thick TiO₂ phases, including rutile and anatase, creates challenges for oxygen penetration into the sample interior. Additionally, the large grain size of anatase reduces the number of grain boundaries, which serve as paths for ion conduction, thereby impeding the rapid movement of oxygen ions. The formation of the TiO phase beneath the rutile/anatase phase aligns with observations reported in other long-term thermal oxidation experiments [40, 41].

In the micropattern sample, crystalline Ni₃Ti₂ is observed just above NiTi matrix, not the amorphous Ni₃Ti₂ which is found in the nanopattern sample (Fig. 5a). The crystalline Ni₃Ti₂ is recognized as a metastable phase and its crystal structure is not well-defined. In this study, the diffraction pattern we obtained was similar to Ni₃Ti₂ with cubic structure shown in ref. [54]. Thus we identified it as Ni₃Ti₂ intermetallic phase based on EDS quantitative analysis results and diffraction pattern indexing results. Further, the excess oxygen in Ni₃Ti₂ layer results in the formation of nano-sized TiO₂ particles in the grains as well as in the grain boundaries.

The microstructural evolution of NiTi alloy upon the number of laser irradiation, encompassing changes in surface roughness, oxide layers and phase transformations is summarized in Fig. 6. With an increasing number of laser irradiations, the surface roughness also increases, leading to the designations "nanopattern" and "micropattern."

Upon a single laser irradiation, the native Ti oxide layer thickens, and the residual Ni forms crystalline Ni nanoparticles in Ti oxide. Additionally, the Ni₃Ti₂ amorphous layer also thickens to approximately 160 nm. Below this layer, a B2 structure is formed as the B19’ martensite undergoes a transformation due to excess Ni content. Repetitive laser irradiations lead to the development of TiO oxide layers, encompassing rutile, anatase, and TiO, with layered structures. On the top surface, Ni expelled from the TiO forms an exceedingly thin oxide layer. Despite the low laser power and scanning speed, the repetitive use of high-frequency laser scanning allows for the formation of anatase as a Ti oxide. At the interface between anatase and Ni oxide, Ni²⁺ probably acts as nucleation site of rutile and promote the phase transformation [55]. The lower region, however, does not experience sufficient oxygen diffusion from the surface, resulting in the formation of TiO. As a result, 4 oxide layers are evolved under repetitive laser irradiation. Furthermore, the Ni₃Ti₂ layer, formed at a considerable depth from the surface and no longer subjected to rapid cooling rates, no longer remains in an amorphous state.

In this study, we maintain a relatively low laser power while varying the number of laser irradiations under high repetition frequency conditions. We not only demonstrate the control of surface roughness but also propose that the type of surface oxide can change with the number of irradiations. Consequently, it is essential to consider how these surface structural alterations may impact different applications within the field of biomedical materials, depending on the intended site of use.