Originally, optical coatings were deposited by evaporating compounds using heat-resistant or electron-beam evaporation sources. The resulting coatings are all non-stoichiometric. To increase the stoichiometric ratio and thus the refractive index of the coating, reactive evaporation techniques were subsequently used. Next, plasma-enhanced evaporation techniques were developed to gain better control over stoichiometry. A technique known as the activated reactive evaporation process, discussed earlier, has been used to deposit a variety of optical films. High-threshold optical thin films of TiO 2 , ZrO 2 and HfO 2 were synthesized.
While color is the essential characteristic of any decorative coating, hardness is of secondary importance. When it comes to matching these two aspects, it is often necessary to accept compromises. In the literature, 57 colors are described with significant deviations from golden yellow to brown, purple, red and gray. This was observed in the samples obtained by high temperature sintering, thus very close to the 50:50 composition. According to this data, all compositions of the binary tin-TiO are yellow; with some twitching, the color passes from blue-brown to metallic gray. PVD yielded somewhat different results because of the low substrate temperature and non-equilibrium compositions with large deviations in stoichiometric ratios are readily available. Compositions where the sum of nitrogen and oxygen is less than 50% are pale yellow or even metallic white, while small excesses of these two components increase light absorption to produce dark shades.
X-ray spectroscopy indicated that no true solid solubility occurred during the low-temperature synthesis. Presumably, excess nitrogen or oxygen is not incorporated into the basic cubic structure, but deposited at defect sites that highly perturb the lattice and grain boundaries, thereby increasing light absorption.
The reflection spectrum of ZrN is obtained with tin coating and magnetron sputtering, as shown in Figure 32.13. The 58 tight meter ZrN coating has a golden yellow color. Tin-based coatings with increased nitrogen content were investigated. Two effects were observed: an excess of nitrogen reduces the reflectance on the long-wave side; the coating turns dark yellow (old gold) and then bronze. The near-UV reflectance minima of the yellow sample are shifted to longer wavelengths. Added oxygen combines in the same direction as excess nitrogen, reducing the red side reflectance to very low values. Due to the minimal displacement, the reflectivity of the blue square increases, neutralizing the red and yellow components and giving the ink, even blue, a tint. The effect of adding carbon is basically to make the dark black by further reducing the total reflectance. Recently, some other binary and ternary systems have been explored, when titanium is replaced by aluminum and vanadium.
The activated reactive ion plating process is used to produce dense, dense nitride, carbide and oxide films at lower substrate temperatures. They are used commercially to produce thin films for optical applications.
During ion plating oxidation, no absorption is measured by simple photometry in the high transmittance range. The measured value of the refractive index of the film is close to that of the material. In all cases, the refractive index is much higher than that of the evaporated film. Table 32.6 lists various comparison values.
Optical properties remain unchanged even during repeated heat treatment cycles. Heating the ZrO2–SiO2 and Ta2O5–SiO2 films on the glass for several hours at 400 °C followed by immersion in water for 3 days produced no change in optical and mechanical properties.
Matching optical and mechanical properties requires some compromises. This is especially true when the color darkens from golden yellow tones due to the overstoichiometric composition. From very high values (over 2500 VH) for yellow coatings, the hardness can be as low as 1000 VH for very dark ones. This is still acceptable to prevent friction from rubbing clothing or traditional cleaning products.
Obviously, the adhesion needs to be "satisfactory", ie there should be no peeling, even at deep scratches. In plasma-assisted PVD methods, adhesion is generally good. Adhesion depends not only on the coating, but also on the substrate. The hardest coatings perform poorly when deposited on soft metal substrates because they have very limited resistance to impact and scratches. At the same time, problems with corrosion protection may arise: the coating cannot be deposited continuously or without pinholes, or may be discontinuous after exposure to mechanical damage.
Satisfactory cases are represented by a cemented carbide substrate bonded (depending on the situation) with a hardness of 1500 VH or more, but dark gray in color, coated by golden yellow TiN or ZrN, even harder than this (2500 to 3000 Hao) . Unfortunately, this solution is very expensive.
There is the first attempt to improve the status quo by using low-pressure plasma nitriding to strengthen glow discharge to obtain a hardness gradient on the surface of alloy steel at one micron, and then to further improve hardness and corrosion resistance by depositing TiN and ZrN coatings.
Direct deposition of hard coatings on brass and similar soft alloys can result in dull coatings with poor scratch and shock protection. There are many micron thickness hard chrome primers, deposited by electrochemical methods, that offer a reasonable solution.
Activated reactive ion plating processes are becoming more and more important in optical applications. Ion bombardment during deposition causes a cascade of atomic collisions in the growing film. Recoiling and displaced atoms cause a continuous mixing of atoms and enhance surface migration. This will result in void filling and smooth or graded grain boundaries. Another consequence of ion bombardment is a high concentration of point defects. These structures are frozen under growth conditions, similar to rapid quenching. High compressive internal stresses are a direct consequence of these defects.
在耐磨防护应用中，密度、高粘合强度和压应力是这些薄脆薄膜取得巨大成功的特征。粘接强度是用来补偿由高压缩固有的生长应力引起的屈曲力，而这种应力是作为机械预应力在实际使用时被薄膜上的机械载荷所放松的。这种预应力的结果是，只要基体的变形保持在弹性范围内，这样的载荷不会损伤薄膜。
当薄膜的热膨胀系数小于衬底时，在高于或等于最高期望工作温度的温度下沉积薄膜时，应增加热压缩应力。当操作温度高于室温时，热应力也会松弛，这是通常情况下的情况。
在没有离子轰击的过程中产生的光学薄膜显示了由于0.1和0.2 eV的低热能而导致的冷凝原子和分子的低迁移率的特性。由于没有电流可以在绝缘衬底上流动，因此，对这种衬底或薄膜进行离子轰击，需要完全相同数量的正电荷和负电荷撞击表面的每一点，以达到电荷平衡。
等离子体辅助离子镀利用了负的自偏压电位，这种电位不仅发生在放置在rf电极上的薄绝缘衬底的表面上。在直流等离子体中，电子的平均速度与等离子体中的离子之间的巨大差异导致等离子体的自偏置电势超过10 V。这种偏置加速了正向衬底表面的离子。它们的能量通常不足以引起溅射，但它们至少比蒸汽原子和分子的热能量高50倍，而且比晶体结合能高。
离解是复合膜的一个问题。即使在蒸发，这是温和的PVD工艺，化学化合物分离到一定程度。由于其低附着系数、气体成分可以抽掉，导致沉积薄膜的化学计量组成。在反应沉积中，气体成分不断地被替换。由于氧的高活性，在反应蒸发过程中成功地生产出了氧化物薄膜。氧化物或氧化亚铜作为蒸发材料，和腔室中的氧分压是稳定在10左右–2 PA使用控制进气阀。实际氧化在很大程度上发生在衬底表面。氧的化学吸附速率是反应完成的关键因素。
然而，光学涂层的反应蒸发产生往往仍稍substo ichiometric因而略有吸收。这些薄膜表面粗糙，柱状或海绵状微结构，空隙量大，内表面面积大。作为低密度的结果，这些薄膜的折射率大大低于散装氧化物的值。这些薄膜吸收大气中的水气和其他气体，从而改变折射率和其他物理性质。它们对基体的附着力差，耐磨性和硬度低。在反应蒸发之前，通过将底物加热到大约300°C来改善这些特性是可能的。在这个过程中，衬底加热实际上是一个标准过程。
多年的经验表明，以成膜原子为主的生长薄膜的轰击有许多优点。因此，离子镀与活化反应蒸发相结合的方法是利用阳极或阴极蒸发源活化涂层材料，从而产生优异的结果。活化反应离子镀不仅包括有偏活化反应蒸发过程，而且是沉积耐磨涂层的三种重要的工业PVD工艺。它们是基于电弧放电，即气体放电，其中大部分的电子是由热阴极点的电子发射产生的。点火后，这个热点可以通过离子轰击加热（自持电弧）来维持，也可以由一个独立的能源如加热灯丝（热离子管）启动、维持和定位。沉积在各种金属物体上的工作条件（压力、温度）通常是典型的柱状结构。这种类型的纹理的后果之一是微观粗糙度，这大约与涂层厚度成正比。在装饰涂层的情况下，通常比沉积的薄，例如，在切削工具上，这种现象不明显，但当新鲜表面被手指接触时就会出现。指纹不能擦去擦痕迹简单因为油腻沉积在微下凹的单柱状晶体之间。这种粗糙度表示，在最坏的情况下，高达10%的总涂层厚度，可以大幅降低的条件下，形成纹理的青睐。众所周知，在活化反应离子镀过程中，强烈低能离子轰击为这种织构和更加明亮的表面创造了有利条件。
为了移动金颜色的化学计量氮化物，氮、氧甚至碳的额外量需要加入到涂层中。
当氮化物单独沉积时，最终的氮含量与气体的分压大致成正比。为了获得较深色的氮氧化物，简单的方法是从一个黄色的氮化物和加氧。钛和锆对这种气体的亲和力明显高于氮气，第一气体比金属蒸气更为定量地聚集。一个结果是需要少量的氧气。
Clearly, carbon cannot be introduced in elementary form. One possibility is to use liquid carbon in gaseous form. The hydrogen portion of these molecules splits to leave free radicals. This free radical has a higher chance of generating new organic molecules among itself than metal carbide formation. A series of organic syntheses took place in the plasma, heavily polluting the chamber and condemning the color reproduction to the failure of the next experiment. The key to solving this problem lies in the use of carbon oxides, inevitably giving oxycarbonitrides, and even carbon-solotrons, to provide pumpers that are compatible with these gases.
Obtaining precise quantitative prescriptions for each color is obviously impossible. Parameters suitable for various coating systems require experimental discovery by trial and error. Further development trends point to new binary or ternary compositions, imparting new colors and potentially even better mechanical and chemical properties.
