Diamond-like carbon (DLC) films exhibit a unique combination of environmental and thermal stability, making them suitable for a variety of demanding applications. These amorphous carbon-based materials are characterized by a mixture of sp³ (diamond-like) and sp² (graphite-like) hybridized carbon bonds, which directly influence their mechanical, chemical, and thermal properties. The stability of DLC films under different environmental conditions is a critical factor in their performance, particularly in applications where exposure to high temperatures, oxidation, or corrosive media is a concern.

One of the most notable properties of DLC films is their oxidation resistance. The high fraction of sp³ bonds contributes to their chemical inertness, reducing susceptibility to oxidative degradation. Studies have shown that undoped hydrogen-free DLC films begin to oxidize at temperatures around 300–400°C in air, while hydrogenated DLC films exhibit lower oxidation thresholds due to the thermal instability of C-H bonds. Oxidation typically initiates at defect sites or sp²-rich regions, leading to the formation of CO and CO₂ gases and gradual film degradation. The oxidation rate is influenced by the film’s composition, with higher sp³ content generally improving resistance. However, even highly sp³-rich films eventually degrade when exposed to prolonged high-temperature oxidative environments.

Thermal stability is another critical aspect of DLC films, particularly concerning the temperature-dependent conversion of sp³ to sp² bonds. This transition is a key factor limiting the high-temperature performance of these materials. At elevated temperatures, the metastable sp³ bonds progressively transform into more thermodynamically stable sp² configurations, leading to graphitization. The onset temperature for this conversion varies depending on deposition conditions and hydrogen content. For hydrogen-free tetrahedral amorphous carbon (ta-C), significant sp³-to-sp² conversion begins around 600–800°C in inert atmospheres. In contrast, hydrogenated DLC films undergo structural changes at lower temperatures (300–500°C) due to hydrogen effusion, which destabilizes the sp³ network. The loss of mechanical hardness and increased surface roughness accompanying this phase transition restrict the use of DLC films in applications requiring sustained operation above these temperature limits.

Humid and corrosive environments present additional challenges for DLC films. Their hydrophobic nature and dense microstructure generally provide good resistance to moisture penetration, reducing the risk of hydrolytic degradation. However, prolonged exposure to high humidity can lead to interfacial weakening in some cases, particularly if the film-substrate adhesion is compromised. In corrosive media such as acidic or alkaline solutions, DLC films demonstrate superior performance compared to many conventional coatings due to their chemical inertness. Electrochemical studies have shown that ta-C films exhibit low corrosion currents and high polarization resistance, making them effective barriers against corrosive attack. However, defects or pinholes in the film can serve as initiation sites for localized corrosion, emphasizing the importance of high-quality deposition processes.

The performance of DLC films in harsh environments is also affected by their residual stress, which can influence long-term stability. Highly sp³-rich films often possess significant compressive stress, which may lead to delamination or cracking under thermal cycling or mechanical loading. Stress relaxation mechanisms become more pronounced at elevated temperatures, further complicating high-temperature applications. Strategies to mitigate these effects include optimizing deposition parameters and implementing graded interlayers, though these approaches must be carefully balanced to preserve the desirable properties of the films.

In terms of thermal conductivity, DLC films exhibit moderate values that are typically higher than those of polymers but lower than crystalline diamond. The thermal conductivity is strongly influenced by the sp³/sp² ratio, with higher sp³ content leading to improved heat dissipation. This property, combined with their electrical insulating characteristics in many cases, makes DLC films attractive for electronic applications where thermal management is crucial. However, the thermal conductivity decreases as sp³-to-sp² conversion occurs at high temperatures, limiting their effectiveness in extreme thermal environments.

The environmental stability of DLC films extends to their radiation resistance, particularly against ultraviolet and gamma radiation. The amorphous carbon network shows less degradation under irradiation compared to many polymeric materials, though prolonged exposure can still induce structural changes. This radiation tolerance expands their potential use in aerospace and nuclear applications where conventional materials may fail.

Despite these advantages, several limitations persist for DLC films in high-temperature environments. The irreversible sp³-to-sp² conversion and associated property degradation set practical upper limits for their use. Additionally, thermal expansion mismatch with common substrate materials can lead to interfacial stresses and coating failure during thermal cycling. These factors must be carefully considered when selecting DLC films for applications involving thermal fluctuations or sustained high temperatures.

In summary, diamond-like carbon films offer a compelling combination of environmental and thermal stability, with notable oxidation resistance and performance in humid or corrosive conditions. Their temperature limitations, governed by sp³-to-sp² conversion and hydrogen effusion in hydrogenated variants, define the boundaries of their applicability. While not suitable for extreme high-temperature environments exceeding their thermal stability thresholds, DLC films remain a robust choice for numerous applications where environmental resistance and moderate thermal stability are required. Continued research into understanding and potentially extending their thermal limits without compromising other properties remains an area of scientific interest.