Environmental Sustainability of High-Power Impulse Magnetron Sputtering Nitriding Treatment of CoCrMo Alloys for Orthopedic Application: A Life Cycle Assessment Coupled with Critical Raw Material Analysis

Abstract

CoCrMo alloys are interesting materials for implantable devices due to their favorable mechanical properties, high wear resistance, and good biocompatibility with the human body. Recent studies have demonstrated the possibility to further increase their wear resistance with an innovative approach consisting of nitriding treatments by the High-Power Impulse Magnetron Sputtering (HiPIMS) technique. Given the novelty of this treatment, it is relevant to develop a preliminary sustainability analysis of the processes to highlight the total environmental impact and to evaluate possible strategies to decrease it. Here, a Life Cycle Assessment (LCA) of HiPIMS nitriding treatments of CoCrMo alloys using a tantalum or molybdenum target is presented. The main impact driver in all impact categories was the electrical consumption of the vacuum apparatus and cooling system of HiPIMS instrumentation with a 45–47% and 37–39% contribution for Ta-based, and 39–40% and 41–42% for Mo-based treatments, respectively. Climate Change was found to be the most impacted category, followed by Resource Use both for Mo and Ta nitriding targets. Therefore, some perspectives to enhance the environmental sustainability of the synthesis process have been considered by means of a sensitivity analysis. Moreover, a Critical Raw Material (CRM) assessment is discussed, providing a complete sustainability evaluation of the proposed HiPIMS treatments.

1. Introduction

Technological developments in the orthopedic field linked to prostheses are constantly increasing with the aim of enhancing the biocompatibility and the wear behavior of the employed materials in the facet joints, specifically for hip and knee replacements. Among the valuable materials used in prosthesis construction, CoCrMo alloys are widely applied materials due to their high biocompatibility and resistance to wear inside the human body. Many studies focused on further increasing the wear resistance of these alloys to obtain elevated performances have been conducted [1,2]. Previous works proposed coating treatments of CoCrMo alloys using different deposition techniques employing various materials and alloy compositions. For example, Holeczek et al. investigated the wear and corrosion resistance of DLC-coated CoCrMo for use in medical implants [3], Balagna et al. assessed the effect of tantalum-based thin film coating of CoCrMo substrate on wear resistance [4], Guo et al. evaluated the effect of coating a CoCrMo alloy with graphene oxide and ε-Poly-L-Lysine for antibacterial properties [5], and Sahasrabudhe et al. proposed a 3D printing treatment of CoCrMo composites adding hydroxyapatite to evaluate the wear behavior and ion release in the presence of natural body fluid [6]. Various thermal or annealing treatments to improve coating adhesion and wear resistance were also investigated [7,8,9]. Recent studies reported valuable results in enhancing the wear resistance of CoCrMo alloys subjected to nitriding processes utilizing High-Power Impulse Magnetron Sputtering (HiPIMS) discharge [10,11]. The experiments were carried out at the National HiPIMS Technology Centre at Sheffield Hallam University using an industrial-size apparatus to develop a nitriding process with HiPIMS discharge. AbuAlia et al. evaluated the effects of different titanium nitride coating processes, using both arc evaporation and HiPIMS techniques, on the wear and ion release of the CoCrMo alloy, confirming the improvement in the final functional characteristics using the HiPIMS technique [12,13,14]. Based on these promising results, Zin et al. performed a nitriding process in a lab-scale HiPIMS apparatus, using different target compositions, and evaluating the tribological, structural, and wear features of treated samples [15]. The obtained results confirmed that the HiPIMS nitriding approach can be an optimal choice to improve the functional characteristics of CrCoMo-based alloys, with the advantage of enhanced energy efficiency, more precise process control, and reduced consumption of resources, if compared to traditional methods. The pulsed high-power nature of this technology enables improved ionization of species, leading to shorter treatment times and potentially lower operating temperatures [11,16,17].

As HiPIMS nitriding treatment is proving to be a promising new approach for the production of CoCrMo alloys in the orthopedic field, it is therefore crucial to proceed with an impact assessment to evaluate the environmental hotspots of the process and enhance its environmental sustainability prior to a possible upscaling.

Life Cycle Assessment (LCA) is a quantitative method to evaluate all potential environmental impacts, including relevant emissions and consumed resources [18]). An LCA analysis can cover the entire life cycle of a specific product, from raw material extraction to waste treatment (cradle to grave). However, for scientific purposes, the common approach is the so-called “cradle to gate” analysis, which allows for the identification of the environmentally critical steps in the development of new production or synthesis routes, excluding the use and the end-of-life phases from the overall evaluation. Several studies demonstrated the importance of an early environmental evaluation, also at the lab scale, to minimize the final impact in the case of a pilot and industrial upscaling [19,20,21,22,23]. In this work, a preliminary LCA study on a nitriding treatment by HiPIMS technology of CoCrMo alloys is presented, concerning the most important environmental hot-spots and the possible strategies to minimize their impact. The first phase of this study focused on defining the goal and scope of the LCA analysis. Accordingly, a brief description of the magnetron sputtering apparatus is provided to detail its constituent components and associated technologies for a clearer assessment of their contributions to various environmental impact categories. The HiPIMS operating parameters, including the final nitriding depth and the target composition, were selected based on previous work published by Zin et al. [15]. Among the various targets investigated by Zin et al. [15], the sustainability assessment in this study concerns exclusively tantalum and molybdenum due to their superior performance in enhancing the wear and corrosion resistance of treated CoCrMo alloys.

The innovative application of HiPIMS for surface nitriding, combined with a comparative environmental impact evaluation of tantalum- and molybdenum-based targets, provides new insights into sustainable coating strategies for implantable devices. The LCA was conducted following the definition of system boundaries and the modeling of the life cycle inventory. Based on the obtained results, potential strategies to improve the environmental performance of the nitriding process were identified. Moreover, the sustainability of the supply chain and the criticality of the target materials employed were evaluated through a Critical Raw Material (CRM) assessment following the European Method [24], also evaluating short-term and long-term criticality indices [25]. To the best of the authors’ knowledge, this is one of the first studies integrating a Life Cycle Assessment and Critical Raw Material analysis of HiPIMS nitriding treatments applied to biomedical alloys.

2. Materials and Methods

2.1. System Description: HiPIMS Physical Vapor Deposition Apparatus

The PVD magnetron sputtering with the HiPIMS technology facility used in this study was widely described in previous works [26,27]. The deposition system consists of a spherical vacuum chamber with a total internal volume of about 50 L. The pumping system is composed of a turbomolecular pump (Pfeiffer HiPace 700, Pfeiffer Vacuum, Wetzlar, Germany) coupled with a preliminary rotary pump (Pfeiffer Pascal 2021, Pfeiffer Vacuum, Wetzlar, Germany): it allows for reaching a final pressure of 10⁻⁴ Pa after about 18 pumping hours. When this base pressure is achieved, a baking process at 400 °C is performed to promote substrate degassing. During this step, at first, the pressure rises to 10⁻² Pa, and then it decreases to 10⁻⁴ Pa after about 2 h. During backing, in addition to the vacuum pumps, a chiller is employed for cooling the magnetron source to prevent the Curie temperature of the magnets from being exceeded (Green box MEC 55/WVP, Green Box srl, Piove di Sacco (PD), Italy). Afterwards, a pure nitrogen flux (99.998%) is inlet by a fluxmeter apparatus (MKs Mass-Flow controllers and Mks Multigas controller 647C, MKS Instruments, Andover, MA, USA) to obtain a working pressure of 10⁻¹ Pa. Finally, this step is followed by the actual nitriding plasmo-chemical treatment. Concerning the cathode, the sputtering target was magnetically fixed to a heat exchanger. To improve heat dissipation, a thermal and electrical contact paste (P 832, Meivac INC, Noelle Industries, San Jose, CA, USA) was spread between the target and the heat exchanger. The sample to be treated (a CoCrMo disk with a diameter of 19 mm and an area of 2.85 cm²) was fixed on a rotating disk sample holder with a diameter of 125 mm and facing the target at a 140 mm distance. The effective area of the sample holder available for the nitriding treatment has a diameter of 70 mm and is positioned in its central part, as shown in Figure 1. The CoCrMo substrate was fixed in this active area. Before entering the vacuum chamber, the substrate was cleaned in an ultrasonic bath (CEIA CP104 tank, FIOA International, Arezzo, Italy) using a solution of 50% isopropanol alcohol (99.5%, Alfa Aesar, Ward Hill, MA, USA) by volume and acetone (Absolute, Carlo Erba, Milano, Italy). The substrate was then dried with pure nitrogen.

The first part of the performed sustainability analysis of the HiPIMS nitriding process was focused on the environmental impact evaluation of the tantalum-based treatment (Ta 99.9% target, a disk with a 101.6 mm diameter and a thickness of 3.175 mm, purchased from Kurt J. Lesker). In this case, the corresponding nitriding penetration rate was about 28.6 nm.min⁻¹ by employing a power supply of 1.85 W cm⁻². The process was performed while maintaining the substrate at 400 °C. Substrate heating was carried out by 8 halogen lamps (power supply of 48 V with a maximum of 2 kW) and controlled by PLC using PID control. The temperature was measured using two K-type thermocouples located in proximity of the sample holder. The same experimental conditions and instrument configuration were used when the nitriding treatment was performed using a molybdenum target (a Mo 99.5% disk with a 101.6 mm diameter and a thickness of 3.175 mm, from Kurt J Lesker). In this case, the penetration rate was estimated to be 16.6 nm.min⁻¹ while fixing the power supply at 1.85 W cm⁻².

Zin et al. showed that a ~3 µm nitriding depth is enough to appreciably improve the wear resistance of CrCoMo samples [15]. Using the Ta target and the sputtering conditions mentioned above, this depth was reached after 2.20 h of treatment, whilst using the Mo target, 4 h of treatment were required. The effective nitriding depth was evaluated using Scanning Electron Microscopy (SEM) images coupled with Energy-Dispersive X-ray Spectroscopy (EDS) analyses (Sigma Zeiss FE-SEM, Carl Zeiss AG, Oberkochen, Germany). The SEM cross-sections micrographs are shown in Figure 2.

2.2. Life Cycle Analysis

2.2.1. Goal and Scope and Functional Unit

The goal of this study was to identify the environmental hotspots in a nitriding treatment process of CoCrMo substrates performed by PVD-HiPIMS instrumentation to improve their wear resistance [15].

The aim of the LCA analysis is to supply insights for upscaling this process to industrial production. In the laboratory process optimization phase, all the experimental parameters involved need to be considered to evaluate the environmental loads of different steps. Consequently, these parameters can be modified to obtain the lowest possible environmental impacts. The first step of this work was to perform a Life Cycle Impact Assessment (LCIA) of a tantalum-based nitriding process, followed by an analysis performed on a CoCrMo alloy with a molybdenum-based nitriding treatment. The Functional Unit (FU) used in this work, defined in the LCA international standard ISO 14040/44 [18] as the “quantified performance of a product system for use as a reference unit”, was 3 µm depth nitriding of a CoCrMo substrate on a 2.85 cm² surface area.

The LCIA results were compared to highlight the environmental hotspot and to perform a sensitivity analysis with the aim to reduce the environmental impact of the proposed nitriding treatments.

2.2.2. System Boundaries and Modeling Assumptions

A crucial step to perform a complete Life Cycle Assessment is to evaluate and clearly enunciate the system boundaries of the analyzed process, identifying inputs and outputs included in LCA analysis. Figure 3 shows the system boundaries of the process analyzed in this study.

The following assumptions were taken in modeling the system:

• Some parameters such as infrastructures, manufacturing instrumentation, and their maintenance were neglected in the analysis as they possess a relatively long lifetime and can be used for various purposes, according to the literature [22,28].
• The transportation of the nitrogen gas used in the treatment process was excluded because the production plant of this gas is located close to the laboratory (less than 1 km).
• The deposition chamber venting was carried out with pure nitrogen (50 L).
• As previously reported in the literature for similar PVD synthesis processes [22], the fraction of the N2 gas consumed for the nitriding process can be considered negligible and, thus, the same quantity introduced to the deposition chamber as input was set in the output as the released gas in the air.
• The nitrogen used to dry the cleaned samples in the cleaning process was considered negligible.
• The pumping time required to reach the suitable base pressure was set to 18 h.
• Water used in the cooling system is not considered as it is inside a continuous cycle without losses.
• The functionality of the treated samples was not considered, as analyzed by Zin et al. [15].
• End-of-life treatments of the samples were also excluded according to previous similar works [22,29].
• According to the authors’ knowledge and considering the magnetic configuration of the instrumental apparatus used in this work, assuming a constant nitriding rate, the target can work correctly until it loses approximately 1/3 of its mass due to repeated treatments. Beyond this point, its remaining portion is considered waste that could be sent to waste-recycling facilities (not considered in this work). Based on that, and fixing 3 µm nitriding depth as reported above, the target lifetime was considered as long as 5320 treatment processes using Ta and 389 treatment processes using Mo (see Sputtering target section below) The recycling processes are out of the boundary system and are not included in this LCA analysis, similar to previous works [22,28].
• In this study, the sample holder was used to hold only one substrate sample, while it could host up to eight samples. In the sensitivity analysis, this parameter is investigated as a prospect to reduce the environmental impacts of the nitriding process.
• The impacts associated with the substrate production processes were not included in the proposed LCA analysis.

2.2.3. Inventory Analysis and Data Sources

The parameters of the nitriding process, including the electricity consumption, gases, solvents, wastes, and emissions, are primary data that were collected during HiPIMS treatments conducted at the National Research Council of Italy (CNR) laboratories in Padua between 2023 and 2024. The distances to the locations of solvents, target material, and thermal contact paste suppliers were calculated using Google Maps software (25.24.01.768809388 version).

Secondary data regarding electricity generation, transportation, solvents and gas production, etc. were obtained from the Ecoinvent 3.10 database, with good temporal representativeness (data from 2011 to 2023). The geographical boundary of this study was set as Italy, and whenever possible, average Italian (e.g., electrical country mix) or European (e.g., transport flows) values were used to model the background processes. In the following, the life cycle inventory data is described in detail, referring to the tables reported in the Supplementary Materials.

• Vacuum apparatus and cooling apparatus: Since the system boundaries of this LCA analysis excluded the manufacturing, maintenance, and end-of-life of the instruments, the inventory for vacuum apparatus and cooling apparatus is only related to electrical consumption. Moreover, in the case of the cooling system, the water consumption was not included, because it is used in a continuous circle without losses. Electrical consumption for both apparatuses was directly collected at CNR ICMATE laboratories by means of TIP power meter 3/16 and Voltcraft SEM6000 devices during different nitriding treatment sessions, using the electrical, low-voltage Italian mix from the Ecoinvent 3.10 database (see Tables S1 and S2 in the Supplementary Materials).
• Sputtering target process: Tantalum and molybdenum targets were purchased from Kurt J. Lesker Company (Dresden, Germany). The target materials are claimed to be manufactured via powder metallurgy and sintering, starting from super pure Ta and Mo powders (99.9%). No mechanical machining was needed and thus, no scraps had been produced. The thermal contact paste was employed to connect the target to the magnetron heat exchanger and was purchased from Maivac INC (Noelle Industries, San Jose, CA, USA). When the lifetime of the targets was over and they needed to be replaced, the contact paste was completely removed from the HiPIMS apparatus, and it was treated as waste together with the scrap targets. For the nitriding process, in the case of the Ta target, the target consumption was measured to be 0.02 g for one treated sample (2.85 cm² area, 3 µm depth), and in the case of the Mo target, this value was measured to be 0.19 g. In both treatment processes, 0.99 g of thermal and electrical contact paste was used to fix the target to the magnetron. To model the Ta and Mo targets, as their specific production data is unknown, secondary data from the Econinvent 3.10 database was used. To produce a target, tantalum and molybdenum need to be in powder form. In the case of a Ta target, the exact inventory process for powder production is available in the database, whilst for Mo, the only process flow present in the Ecoinvent is related to molybdenum ingot production and it was used as the material input. The amount of electricity required to obtain Mo powder was taken from an Econinvent dataset representing a typical synthesis process to obtain the powder required in manufacturing sputtering targets (Indium tin oxide powder, ITO). The same process was used as the source regarding the powder transportation of Mo and Ta. The typical sintering temperature of ITO is in the 1500–1600 °C range comparable to that of Ta and Mo (about 1600 °C and 1300 °C, respectively) [30,31]. For this reason, the process for manufacturing sputtering targets from ITO powder was considered a suitable representative for Ta and Mo target sintering, and the required electrical consumption was taken from this unit process. Also, target transportation from the target sintering plant (Kurt J. Lesker, Dresden, Germany) to Padua was by truck and calculated to be 1000 km on average. A complete inventory of the input parameters and the unit processes used to model them is listed in Tables S3 and S4 of the Supplementary Materials. The composition of thermal and electrical contact paste was taken from a previous work where the same paste was employed [22]. The inventory table of thermal and electrical contact paste, and the transportation data from the USA to Italy are reported in the Supplementary Materials (Table S5). The technical data is from its safety data sheet (TP 832, Meivac INC, Noelle Industries). The thermal contact paste was incinerated at the target end-of-life.
• Cleaning CoCrMo substrate: Ultrasonic cleaning was performed in a 40 mL solution composed of 50% v/v acetone and 50% v/v isopropanol. The amount was calculated according to the density of acetone (0.78 g/cm³) and isopropanol (0.8 g/cm³). The electricity consumption was calculated for a cleaning time of 3 min at 400 W. An average distance of 1000 km by lorry (Euro 5) was considered for solvent transport to the CNR laboratory. After the cleaning process, the solvent mixture was discarded and treated as spent solvents (Table S6 in the Supplementary Materials).
• Gas system apparatus: The gas system apparatus is composed of fluxmeters and sensors to evaluate the correct flux of gases used in the surface treatment process. In this specific case, the nitriding process uses only pure nitrogen fed into the vacuum chamber during the substrate treatment. All related data including the electricity used by the fluxmeter and sensors, gas amount, and emissions is reported in Table S7 of the Supplementary Materials.

3. Results and Discussion

3.1. Life Cycle Impact Assessment (LCIA) Results

A Life Cycle Impact Assessment was performed using the Environmental Footprint (EF) 3.1 adapted method, following the European Commission’s recommendation [32,33], and using the Simapro 9.6 software. Table S8 in the Supplementary Materials reports all the impact categories of this method, with respective reference units and a brief description of them. Characterization, normalization, and weighting were carried out for nitriding processes using tantalum and molybdenum targets. Figure 4 and Figure 5 illustrate the results of the contribution analysis of nitriding treatment with Ta and Mo, respectively.

Contribution analyses showed that the main impacting processes are attributable to the “Vacuum system” and “Cooling system”, regardless of the target composition. In particular, in the case of Ta target, the impacts of the vacuum system and cooling system account for 45–47% and 37–39% in all impact categories, respectively. Similarly, when the Mo target was used, the vacuum system presented an impact in the range of 39–40% with respect to the total impact, and the cooling system had an impact of 41–42% in all categories. Figure 6 exhibits a comparison of different impact categories using Ta or Mo targets. The results show that the most impacted category is “Climate Change” for both nitriding processes, followed by Resource Use. In general, using Mo as the target, the impact is always higher than using Ta.

The differences in the impact assessment of the two nitriding processes are correlated to the different duration of the processes using different target compositions: as reported in the inventory section, to obtain a treatment depth of 3 µm (the functional unit of the study), the treatment takes 4 h in the case of Mo target, while it is 2.2 h in the case of Ta target. The electrical consumption of the cooling system is linear with respect to time for different steps of the HiPIMS process, namely, the pumping period to reach the desired pressure, the treatment itself, and the period when the temperature decreases from 400 °C to 100 °C. Regarding the vacuum system, the total amount of energy used is about 10,000 Wh for both treatments, with the major consumption attributed to the evacuation time for the vacuum pump to reach the correct pressure for HiPIMS treatments. Consequently, a nitriding treatment of 4 h consumed only 700 Wh more energy than a 2.2 h one. This result indicates that a longer nitriding treatment does not significantly affect the total energy consumption (Table S1 in the Supplementary Materials).

Furthermore, the two targets showed distinct effects on the Resource Use, Minerals and Metals impact category, with Mo-based nitriding characterized by a larger impact due to the significant use of target material. As reported before, to obtain a treatment depth of 3 µm, 0.19 g of Mo is required, while lower amounts are needed for a treatment based on Ta (0.02 g), due to its higher sputtering rate. The impact of using different target compositions was considerable, mainly in the case of the Resource Use, Minerals and Metals category (followed by the Ecotoxicity Freshwater, Eutrophication Freshwater, and Human toxicity-non cancer categories), reiterating the fact that the main environmental loads are associated with the vacuum system and cooling system processes.

Figure 7 exhibits the “single score” results of the present LCA analysis with the EF 3.1 method comparing the treatments using tantalum or molybdenum. The single score is an end-point indicator obtained by characterization, normalization, and weighting of the midpoint indicators [34]. It can be clearly observed that the vacuum system and cooling system have the main impact for both nitriding processes, as previously shown in the contribution analysis.

In order to address the sustainability of the HiPIMS nitriding approach, we performed a complete sensitivity analysis, evaluating the experimental conditions that could potentially lower environmental impacts. Due to the vacuum chamber configuration of the HiPIMS apparatus used in this work, it is possible to make a simultaneous treatment of eight samples with an area of 2.85 cm² (or of a larger substrate with a total area of 38.5 cm²) without changing both deposition conditions and without increasing target consumption. Consequently, the environmental impact related to the functional unit chosen (3 µm depth for 2.85 cm² area) will decrease linearly as shown in Figures S1 and S2 in the Supplementary Materials.

As the gas system demonstrated a relatively low environmental impact, the possibility of using less active gas for the nitriding treatment cannot be an effective option to improve the environmental performance of the process. Similarly, substrate cleaning also shows a low impact, and thus, performing the cleaning simultaneously for several substrate samples using the same quantity of solvents used for cleaning one substrate will not considerably change the final environmental impact.

Conversely, reducing the pumping time required to reach the desired vacuum pressure could be a viable option. This duration can be decreased from 18 h to 6 h, obtaining a vacuum value (<10⁻³ Pa) suitable to perform nitriding treatment, as confirmed by previous lab campaigns, and which would be extremely effective in lowering the impacts associated with vacuum system electrical consumption. As reported in the Supplementary Materials, using the tantalum target and reducing the pumping time from 18 h to 6 h, the results showed a decrease of about 22% for all impactful categories. Similarly, using the molybdenum target, the pumping time reduction caused a decrease of about 18% in all categories. For the complete results of this sensitivity analysis, see Figures S1–S4 in the Supplementary Materials.

The results of the sensitivity analysis underlined the importance of exploring strategies to reduce the electrical consumption of the proposed technique, especially regarding the vacuum system. These findings align with previous studies [22] that also pointed out the significant energy impact of vacuum equipment in coating processes. Given that the vacuum system was found to be the main contributor to energy use in the HiPIMS setup adopted here, identifying efficient solutions will be key for any potential industrial scale-up.

3.2. Critical Raw Material (CRM) Assessment

We also performed a CRM assessment of the raw materials selected as the target composition in the nitriding processes to improve the sustainability analysis. The CRM assessment can be performed following different proposed methodologies, namely, a global approach using the Yale method, a technology approach using the US DoE or JRC methods, or an economical one, performed using the EU CRM assessment and BRGM and BGS methods [35]. The two factors evaluated in the EU CRM method are the ”Supply Risk“ (SI), the possibility of disruption in the supply of a specific material, and the ”Economic Importance“ (EI), which indicates the importance of a material in the European economy. The supply risk of a specific material, mineral, or element is evaluated using the following formula:

$$\mathrm{S}\mathrm{R}=\left[{\left(\mathrm{H}\mathrm{H}{\mathrm{I}}_{\mathrm{W}\mathrm{G}\mathrm{I},\mathrm{t}}\right)}_{\mathrm{G}\mathrm{S}}\times \mathrm{I}\mathrm{R}/2+{\left(\mathrm{H}\mathrm{H}{\mathrm{I}}_{\mathrm{W}\mathrm{G}\mathrm{I},\mathrm{t}}\right)}_{\mathrm{E}\mathrm{S}}\times \left(1-\mathrm{I}\mathrm{R}/2\right)\right]\times \left(1-\mathrm{E}\mathrm{O}{\mathrm{L}}_{\mathrm{R}\mathrm{I}\mathrm{R}}\right)\times \mathrm{S}{\mathrm{I}}_{\mathrm{S}\mathrm{R}}$$

(1)

in which the HHI_WGI,t parameter is the Herfindahl–Hirschman Index (the HHI emphasizes the role of large global or European suppliers of the material and their share in production), weighted by the World Governance Indicator (WGI, an indication of the quality of governance at the national level), and corrected by the Trade Parameter (t). EOL_RIR (end-of-life recycling input rate) measures recycling contribution to material demand. The SI_SR (substitutability index) shows the availability of the substitute raw material and the extent to which it can be used, and IR indicates the degree of import reliance. The economic importance is the other factor and is calculated as follows:

$$\mathrm{E}\mathrm{I}={\sum }_{\mathrm{i}}\left({\mathrm{A}}_{\mathrm{i}}\times {\mathrm{Q}}_{\mathrm{i}}\right)\times {\mathrm{S}\mathrm{I}}_{\mathrm{E}\mathrm{I}}$$

(2)

where A_i is an indication of the demand for the material in a particular EU economy sector and Q_i shows the corresponding added value to that sector. Moreover, the SI_EI index reflects the alternative raw materials that can supply the same function, considering the cost and performance of the substitute with respect to the current practice. Using the supply risk and economic importance formula, the corresponding values were calculated in the case of molybdenum (EI = 6.7, SR = 0.8) and tantalum (EI = 4.8, SR = 1.3). The threshold values of EI and SR have been identified by the European Commission and are 2.8 and 1, respectively. A material is considered critical if its parameters exceed both indexes [24]. Noteworthy, Mo is critical only in the case of economic importance, while Ta exceeds both thresholds. In fact, tantalum is identified as a critical material by the EU CRM list. The end-of-life recycling input rates of Mo and Ta are reported as 30% and 1%, respectively, in the latest CRM study of the European Commission [36]. While the EOL_RIR is considered a mitigation factor in the Supply Risk index, the lower recycling value of tantalum results in a higher SR index compared to molybdenum. In nature, tantalum cannot be found as a free metal but in a complex mineral matrix, often in combination with niobium [37]. Tantalum has a broad application field, but it is mainly used in Ta capacitors that are essential for emerging technologies, including PWBs (Printed Wiring Boards) for self-driving cars. Tantalum as a natural resource is estimated to have a lifetime of less than 50 years based on the current extraction annual rate [38]. Tantalum-recycling processes and efforts to improve their recycling rates are becoming important topics of research due to the scarcity in natural resources. For this reason, industrial companies are investigating different recycling processes, including the use of spent Ta sputtering targets as raw materials (e.g., the Global Advanced Metal Company, Boyertown, PA, USA, and Tantalum Recycling Company, Doral, FL, USA). The same waste treatment considerations are also valid for molybdenum targets, with a relatively larger number of companies that offer recycling services. It is important to highlight that the proposed HiPIMS nitriding treatments are characterized by a very low target consumption, which is of relevance, especially in the case of scarce materials on the Earth’s crust. As reported above, in magnetron sputtering treatments, the target can be used only partially before it needs to be replaced. In general, a waste between 2/3 and 1/2 of the original target weight occurred. However, the unused target is not contaminated during the sputtering process, and for this reason, it could be easily recycled. Moreover, with the HPIMS configuration used in this work, the high sputtering rate of the tantalum target led to a very low use of raw material compared to molybdenum.

The Resources Scanner (www.grondstoffenscanner.nl accessed on 20 April 2025) is another tool to perform economic-wide criticality assessments considering the European Union context, permitting the evaluation of the long-term and short-term security of the supply of elements calculated according to the equations reported by Bastein & Rietveld [25]:

$$\mathrm{C}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{t}{\mathrm{y}}_{\mathrm{L}\mathrm{T}}=\mathrm{H}\mathrm{H}{\mathrm{I}}_{\mathrm{r}\mathrm{e}\mathrm{s}}+\mathrm{P}/\mathrm{R}+\mathrm{C}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}$$

(3)

and

$$\mathrm{C}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{t}{\mathrm{y}}_{\mathrm{S}\mathrm{T}}=\mathrm{H}\mathrm{H}{\mathrm{I}}_{\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}}\times \left(\mathrm{W}\mathrm{G}{\mathrm{I}}_{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{e}\mathrm{d}}+\mathrm{O}\mathrm{C}\mathrm{E}{\mathrm{D}}_{\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{e}\mathrm{d}}\right)\times \left(1-\mathrm{E}\mathrm{O}{\mathrm{L}}_{\mathrm{R}\mathrm{I}\mathrm{R}}\right)$$

(4)

where HHI_res indicates the concentration of raw material reserves, P/R considers the risk from proven low reserves and is the ratio between production and reserves, Companionality is the degree to which raw material is a by-product, HHI_prod represents the concentration of raw material production (or extraction), and OCED shows the proportion of the raw material in global production that has been affected by prohibitions or restrictions, during the past five years. The results of calculating the short- and long-term criticality of molybdenum and tantalum are reported in

Table 1. Calculation of the long-term and short-term criticality of molybdenum and tantalum.

	P/R	Comp.	HHIres	Crit. LT	HHIprod	WGI	OECD	EOLRIR	Crit. ST
Ta	0.08	0.28	0.3	0.66	0.17	0.12	0.51	0.06	0.10
Mo	0.04	0.46	0.27	0.77	0.26	0.24	0.42	0.33	0.11

.

Comparing the long-term criticality of the two elements, the main significant difference is the Companionality index. This is because Mo is often produced as a by-product of another process (copper mining). The HHI_res index is practically the same for both elements (0.3 for Ta and 0.27 for Mo), and their P/R value is not significantly different (0.08 for Ta and 0.04 for Mo) due to proven low geological reserves of both elements. Regarding short-term criticality, the main differences between the two analyzed elements are related to the EOL_RIR and WGI indices. As previously reported, Ta suffers from a low recycling rate, determining a value of 0.33 of EOL_RIR with respect to a 0.06 value for Mo. Moreover, Ta has high concentration in countries with poor qualities of governance (high WGI), compromising its supply chain stability. On the contrary, Mo presents high values of HHI_prod index, related to the fact that Mo production occurs in fewer countries, setting a risk of monopoly formation. The balance of all these parameters determines a similar short-term criticality for Mo and Ta (0.10 for Ta and 0.11 for Mo).

4. Conclusions

In this work, a combined approach of Life Cycle Assessment and Critical Raw Material Analysis was applied to evaluate the sustainability of HiPIMS nitriding treatments aimed at improving the wear resistance of CoCrMo alloys. The Life Cycle Impact Assessment identified that the main impact driver was electrical consumption during the nitriding treatment related to the vacuum system apparatus and the cooling system apparatus. In order to identifying the best strategy to improve the environmental sustainability of the presented nitriding approach, a sensitivity analysis was performed, showing that the total environmental impact can be decreased, reducing the pumping time required to reach the needed vacuum chamber conditions. For all impact categories, a reduction of about 22% using the tantalum target and reducing the pumping time from 18 h to 6 h was obtained. Similarly, using the molybdenum target, reduced pumping time causes a decrease of about 18% in all categories. The use of a specific target composition can also affect the total impact of the proposed nitriding treatment. In this case, the environmental impacts of using tantalum and molybdenum targets were characterized and found to be considerable, mainly in the case of the Climate Change and Resource Use, Minerals and Metals categories. In general, using Mo, the impact was higher than when using Ta in all impact categories. This is associated with the significant use of Mo as the target material due to its relatively lower sputtering rate. Moreover, a Critical Raw Material assessment following the EU CRM method was performed with the aim of evaluating the criticality of Ta and Mo in terms of supply risk and economic importance. The results revealed that, although tantalum may be preferable in terms of minimizing environmental impact, its criticality, limited recyclability, and supply risk need to be carefully considered in future-oriented developments. Thus, combining LCA analysis with CRM assessment can be a useful tool to support decision-making related to resource efficiency in business and governance to monitor the use of raw materials, analyze the environmental performance of possible alternative materials, prioritize critical materials with respect to each other, and evaluate trade-offs between environmental impacts of using different materials in a specific application.