Control of tribological properties of diamond-like carbon films with femtosecond-laser-induced nanostructuring

This paper reports tribological properties of diamond-like carbon (DLC) films nanostructured by femtosecond (fs) laser ablation. The nanostructure was formed in an area of more than 15 mm × 15 mm on the DLC surface, using a precise target-scan system developed for the fs-laser processing. The frictional properties of the DLC film are greatly improved by coating a MoS₂ layer on the nanostructured surface, while the friction coefficient can be increased by surface texturing of the nanostructured zone in a net-like patterning. The results demonstrate that the tribological properties of a DLC surface can be controlled using fs-laser-induced nanostructuring.     

Fabrication of diamond-like carbon thin films by femtosecond laser ablation of frozen acetone

Diamond-like carbon (DLC) thin films were fabricated by the ablation of frozen acetone with a 790 nm, 130 fs Ti:sapphire laser. Compared to a solid carbon target, frozen acetone could significantly reduce the number of fragments mixed into the films. The optical and mechanical properties of the fabricated DLC films were determined when the laser fluence was varied from 3 to 470 J/cm². With the increase in laser fluence, the films tinged with brown and the optical bandgap of the films decreased from 2.0 to 1.2 eV. Also, the refractive index and hardness of the films increased from 1.75 to 1.99 and from 10 to 16 GPa, respectively. The sp³ content was not changed even if the laser fluence was varied. The change in properties resulted from the hydrogen content of the films. PACS 81.05.Uw; 81.15.Fg     

Laser-induced spallation in diamond-like carbon films

Laser-induced multilevel spallation in diamond-like carbon films is observed and studied for a wide range of laser parameters (=539, 800, 1064, and 1078 nm; =100 fs, 300 ps, 10 ns, and 150 ns; E>0.1 J/cm²). Laser irradiation causes the detachment and removal of material layers, whose lateral size is comparable with the irradiated area and whose thickness of 10–200 nm is much less than the DLC film thickness. The influence of the laser parameters on the thickness, size, and number of removed layers is examined. A qualitative explanation of laser-induced spallation in DLC films is proposed, which is based on the fact that the absorbed laser pulse produces a rapidly expanding surface layer of modified structure and reduced density. PACS 42.62.Cf; 61.80.Ba; 81.40.Wx     

飞秒激光制备硅窗口增透保护类金刚石膜

采用飞秒激光(800 nm,120 fs,3 W,1 000 Hz)制备类金刚石膜,研究了不同偏压、生长温度和氧气氛等辅助手段对激光沉积类金刚石膜的影响,实验发现在室温(25℃)、无偏压和低气压氧气氛(2 Pa)条件下沉积的类金刚石膜性能最优。在单面预镀普通增透膜的硅红外窗口材料上镀制出了无氢类金刚石膜,3~5μm波段平均透过率达到90%以上,纳米硬度高达40 GPa,用压力为9.8 N的橡皮磨头,摩擦105次,膜层未见磨损,并且通过了军标规定的高温、低温、湿热、盐雾等环境试验,所制类金刚石膜可对红外窗口起到较好的增透保护作用。     

横向电场下的类金刚石薄膜激光损伤

类金刚石(DLC)薄膜在红外区有很高的透过率,但激光损伤阈值低,严重限制了其应用领域。采用直接在DLC薄膜上沉积Ti电极,基于激光损伤阈值(LIDT)测试平台,用1-on-1零几率损伤法,研究了在不同偏置电场下DLC薄膜损伤阈值及损伤形貌的变化。发现电场强度从0增加到700V/cm,损伤阈值明显增大;进一步增大偏置电场,损伤阈值相对不变。分析认为偏置电场改变了激光辐照DLC薄膜区域的光生载流子漂移速度,减小了DLC薄膜的局部热累积,减缓了薄膜的石墨化进程,提高了DLC薄膜的抗激光损伤阈值。     

Diamond-like phase formation in an amorphous carbon films prepared by periodic pulsed laser deposition and laser irradiation method

Diamond-like carbon (DLC) films were fabricated by pulsed laser ablation of a liquid target. During deposition process the growing films were exited by a laser beam irradiation. The films were deposited onto the fused silica using 248 nm KrF eximer laser at room temperature and 10⁻³ mbar pressure. Film irradiation was carried out by the same KrF laser operating periodically between the deposition and excitation regimes. Deposited DLC films were characterized by Raman scattering spectroscopy. The results obtained suggested that laser irradiation intensity has noticeable influence on the structure and hybridization of carbon atoms deposited. For materials deposited at moderate irradiation intensities a very high and sharp peak appeared at 1332 cm⁻¹, characteristic of diamond crystals. At higher irradiation intensities the graphitization of the amorphous films was observed. Thus, at optimal energy density the individual sp³-hybridized carbon phase was deposited inside the amorphous carbon structure. Surface morphology for DLC has been analyzed using atomic force microscopy (AFM) indicating that more regular diamond cluster formation at optimal additional laser illumination conditions (∼20 mJ per impulse) is possible.     

离子束流后处理对类金刚石薄膜激光损伤特性的影响

类金刚石薄膜激光损伤阈值低,已经严重制约其在红外激光系统中的应用。基于非平衡磁控溅射技术,在硅基底上沉积类金刚石薄膜;采用离子束流后处理技术,用正交实验法确定影响处理效果的主要因素,对已沉积完成的DLC薄膜进行离子束轰击;在不同处理工艺下,观测薄膜样品的光学常数及拉曼光谱,最后进行了激光损伤测试。从测试结果可知,离子束流后处理参数:离子能量1000eV、放电电流30~40mA、轰击时间8min时,透射率由原来的60.65%提高到了65.98%;消光系数在900nm后明显降低,DLC薄膜的激光损伤阈值从0.69J/cm2提高到1.01J/cm2。     

脉冲激光对类金刚石（DLC）薄膜的热冲击效应研究

强激光辐照红外热像系统时，可造成系统的干扰和破坏，激光的波长不同，对系统的破坏效果也不同.为了保护红外系统窗口以及提高窗口的透过率，红外窗口广泛沉淀类金刚石（DLC）薄膜.当入射的激光波长位于红外系统响应波段外时，激光对系统的破坏首先是激光对DLC薄膜的破坏.以波长为1.06μm的激光为例，研究了脉冲激光对DLC薄膜的损伤机理，建立了DLC薄膜的热冲击效应模型，并通过求解热传导和应力平衡方程，得出了薄膜的温度场和应力场分布.理论分析表明，热应力破坏在脉冲强激光对DLC膜的损伤机理中占主导地位.当 辐照能量密度为E₀=100mJ·cm^-2时，在薄膜表面距光斑中心约 40μm区域内的压应 力明显超出其断裂强度，将造成膜层的剥离、脱落.理论分析与实验结果基本相符，表明建 立热冲击效应模型的正确性. 关键词： 激光辐照 类金刚石（DLC）薄膜 热冲击效应     

Fe纳米颗粒嵌埋对类金刚石薄膜结构及电学性能的影响

 采用脉冲激光气相沉积方法制备了不同Fe嵌埋浓度的Fe: DLC多层纳米复合薄膜。用X射线光电子能谱仪(XPS)对薄膜的组成成分进行分析。利用透射电子显微镜（TEM）、拉曼光谱、电流-电压曲线研究Fe纳米颗粒嵌埋对薄膜的微观结构及电学性能的影响。XPS和TEM表明,Fe纳米颗粒周期性地均匀地嵌埋在碳薄膜中。拉曼光谱表明薄膜中的C为典型的类金刚石结构,Fe纳米颗粒促进芳香环式结构的形成,薄膜结构的有序度提高。电流 电压曲线表明,Fe纳米颗粒的嵌埋导致薄膜的室温电导率增加。     

超短脉冲准分子激光淀积类金刚石薄膜的实验研究

利用ＫｒＦ超短脉冲激光器开展了超短脉冲激光淀积金刚石薄膜实验研究,薄膜生长速率０．０２ｎｍ／ｐｕｌｓｅ,厚度０．５－０．６μｍ,显微硬度５５ＧＰａ,光学透过率优于９０％,采用等离子本的时间－空间分辨的发射光谱技术,给出了等离子体的粒子成份以及等离子体的时间－空间的演化图像,以及在不同激光参数条件下等离子体特性的变化,发现超短脉冲激光的等离子体有能量高、持续时间短（高通量）的特点,适合类金刚石薄膜的     

类金刚石薄膜及其进展

为了使研究者能更详细地了解类金刚石（DLC）薄膜的研究现状,综述了类金刚石薄膜的特性及应用,分析对比了目前常用的一些类金刚石薄膜的制备方法,包括物理气相沉积法（PVD）和化学气相沉积法（CVD）,并对类金刚石薄膜的抗强激光损伤特性以及提高其激光损伤阈值的方法进行了论述。结果发现,利用PVD法制备的DLC膜的硬度可以达到40 GPa~80 GPa,且薄膜的残余应力可以达到0.9 GPa~2.2 GPa之间,而CVD法则由于反应气体的充入导致类DLC薄膜的沉积速率大大降低,故使用率不高。同时,优化膜系的电场强度设计,采用合理的制备工艺,进行激光辐照后处理,施加外界电场干预均可有效地提高DLC薄膜的抗激光损伤能力, 且目前的DLC薄膜的激光损伤阈值可达到2.4 J/cm2。     

Fabrication and characterization of freestanding ultrathin diamond-like carbon targets for high-intensity laser applications

Here, we report the fabrication of diamond-like carbon (DLC) thin films using pulsed laser deposition (PLD). PLD is a well-established technique for deposition of high-quality DLC thin films. Carbon tape target was ablated using a KrF (248 nm, 25 ns, 20 Hz) excimer laser to deposit DLC films on soap-coated substrates. A laser fluence between 8.5 and 14 J/cm² and a target to substrate distance of 10 cm was used. These films were then released from substrates to obtain freestanding DLC thin foils. Foil thicknesses from 20 to 200 nm were deposited using this technique to obtain freestanding targets of up to 1-inch square area. Typically, 100-nm-thick freestanding DLC films were characterized using different techniques such as AFM, XPS, and nano-indentation. AFM was used to obtain the film surface roughness of 9 nm rms of the released film. XPS was utilized to obtain 74 % sp², 23 % sp³, and 3 % C–O bond components. Nano-indentation was used to characterize the film hardness of 10 GPa and Young’s modulus of 110 GPa. Damage threshold properties of the DLC foils were studied (1,064 nm, 6 ns) and found to be 7 × 10¹⁰ W/cm² peak intensity for our best ultrathin DLC foils.     

Characterization of the bonding structure of nanocrystalline diamond and amorphous carbon films prepared by plasma assisted techniques

Thin nanocrystalline diamond/amorphous carbon (NCD/a-C) composite films and amorphous diamond-like carbon (DLC) films were prepared by three methods: microwave plasma chemical vapour deposition (MWCVD) from methane/nitrogen mixtures (NCD/a-C), RF magnetron sputtering of a pure graphite target in argon/methane ambients, and pulsed laser deposition (PLD) in vacuum or argon atmosphere (DLC). The films prepared by the three techniques were comprehensively characterized with respect to their bonding structure by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). PACS 81.05.Uw; 82.80.Pv     

Optical properties of La2O3 doped diamond-like carbon films

A novel kind of La₂O₃ doped diamond-like carbon (DLC) films with thickness of 100-120 nm were deposited by unbalanced magnetron sputtering. Raman spectra and photoluminescence properties were measured by Raman spectrometer operated by 325 nm He-Cd laser and 514 nm Ar⁺ laser, respectively. The intensities of Raman spectra and photoluminescence are higher than those of pure DLC films. The La₂O₃ doped DLC films have the potential promising for the application of solar cell coatings.     

Improving the conductivity of diamond-like carbon films with zinc doping and its material properties

Zinc doped diamond-like carbon (DLC) nanocomposite thin films are fabricated by KrF pulsed laser deposition. Carbon targets containing 3.0, 5.0 and 10.0 atomic percentages of zinc are used as the source for the laser system. Investigation of electrical properties by the four-point probe shows that doping zinc into DLC can lower the electrical sheet resistivity. Microstructural analysis by Raman spectroscopy and XPS show a lower sp³ content but a higher SiC content with an increasing amount of zinc incorporation. The increase of SiC leads to an increase in adhesion strength. Surface roughness of the films also increases while the coefficients of friction for the films do not change.     

外加偏置电场下类金刚石薄膜激光损伤形貌

使用非平衡测控溅射技术沉积了类金刚石薄膜,对比了外加偏置电场前后薄膜的抗激光损伤表面形貌变化,发现薄膜施加偏置电场后,薄膜的激光损伤区域内有大量丝状薄膜,损伤形貌存在明显不同,损伤面积减小,薄膜的激光损伤情况得到改善。这表明外加偏置电场对薄膜的损伤有影响,激光在薄膜中激励产生的光生电子在电场作用下产生快速漂移,间接降低了激光辐照区域内的局部能量密度,减缓了薄膜的石墨化,提高了薄膜的抗激光损伤能力。     

Influence of sulfidation treatment on the structure and tribological properties of nitrogen-doped diamond-like carbon films

The nitrogen-doped diamond-like carbon (DLC) films were deposited on high speed steel (HSS) substrates in the direct current unbalanced magnetron sputtering system. Sulphurized layer was formed on the surface of DLC films by means of liquid sulfidation in the intermixture of urea and thiourea solution in order to improve the tribological properties of DLC films. The influence of sulfidation treatment on the structure and tribological properties of DLC films was investigated in this work. The structure and wear surface morphology of DLC films were analyzed by Raman spectroscopy, XPS and SEM, respectively. It reveals that the treated films are smooth and uniform; and sulfur atoms are bonded chemically. The treated films have broader distribution of Raman spectra in the range of 1000-1800 cm⁻¹ and higher I_D/I_G ratio than the untreated films as a result of the appearance of the crystalline graphite structure after the sulfidation treatment. It is showed that the sp² relative content increase in the treated films from the XPS measurement. The Raman results are consistent with the XPS results. The tribological properties of DLC films were investigated using a ball-on-disk rotating friction and wear tester under dry friction conditions. It is found that the sulfidation concentration plays an important part in the tribological properties of the treated DLC films. The results showed the treated films with low sulfidation concentration have a lower friction coefficient (0.1) than the treated films with high sulfidation concentration (0.26) and the untreated films (0.27) under the same friction testing conditions, which can be attributed to both the presence of sulfur-containing materials and the forming of the mechanical alloyed layer on the wear surface. Adding the dry nitrogen to the sliding surface in the testing system helps the friction coefficient of the treated films with low sulfidation concentration to decrease to 0.04 further in this work. On the basis of the experimental results, it is indicated that the liquid sulfidation technique, which is low-cost, non-polluting and convenience, would be an appropriate method for the surface treatment of DLC films.     

Reflectivity in femtosecond-laser-induced structural changes of diamond-like carbon film

We report characteristic changes in reflectivity of diamond-like-carbon (DLC) films following irradiation of femtosecond laser pulses at laser fluence around the ablation threshold, where we have observed the formation of periodic nanostructure on the film surface and the modification of bonding structure from DLC to glassy carbon in our recent experiments. The experimental results have shown that the reflectivity change should be a reliable measure to monitor the experimental condition to produce the structural changes, being closely associated with the fundamental physical process to induce these phenomena. PACS 79.20.Ds; 81.40.Tv; 42.62.Cf     

Amorphous hydrogenated carbon synthesized by pulsed laser deposition from cyclohexane

It is demonstrated that a liquid hydrocarbon precursor, cyclohexane, is appropriate for laser-induced carbon deposition. Amorphous hydrogenated carbon films (a-C:H) were deposited by KrF excimer laser irradiation of single-crystal silicon surface immersed under cyclohexane. The technique is simple and easy to operate. IR absorption spectra of the deposited films confirmed the presence of carbon in the diamond phase. Raman and XPS studies showed diamond-like character of the deposited films. Moreover, these two studies provided strong evidence that laser fluence played an important role in the formation of DLC bondings and the quality of the deposited films. Received: 15 September 1998 / Accepted: 5 January 1999 / Published online: 5 May 1999     

Atomic oxygen resistant behaviors of Mo/diamond-like carbon nanocomposite lubricating films

Mo doped diamond-like carbon (Mo/DLC) films were deposited on Si substrates via unbalanced magnetron sputtering of molybdenum combined with plasma chemical vapor deposition of CH₄/Ar. The microstructure of the films, characterized by transmission electron microscopy and selected area electron diffraction, was considered as a nanocomposite with nano-sized MoC particles uniformly embedded in the amorphous carbon matrix. The structure, morphology, surface composition and tribological properties of the Mo/DLC films before and after the atomic oxygen (AO) irradiation were investigated and a comparison made with the DLC films. The Mo/DLC films exhibited more excellent degradation resistant behaviors in AO environment than the DLC films, and the MoC nanoparticles were proved to play a critical role of preventing the incursion of AO and maintaining the intrinsic structure and excellent tribological properties of DLC films.