Vojnotehnicki glasnik / Military Technical Courier

5-16

Authors: Mrdak Mihailo R.

The paper presents the results of the examination of atmospheric plasma spraying of APS-NiCr/Al coatings. Composite NiCr/Al powders, composed of NiCr particles coated with fine Al particles, enable an exothermic reaction during the deposition process. The exothermic reaction of these metals results in the formation of selfbonding NiCrAl coatings with a unique combination of properties. For the purpose of obtaining excellent structural and mechanical properties, the deposition parameters were optimized. In this study, three groups of NiCrAl coating samples with different powder deposition rate were made to get deposits of greater strength and toughness. In the first group of samples, the deposition rate was 50g/min, in the second group the powder deposition rate was 40 g/ min. while in the third group it was 30 g/min. The assessment of the properties of deposits was made by the HV0.3 microhardness testing method and the bond strength was examined by tensile strength testing. The metallographic evaluation of the structure was done by optical microscopy The verified coatings were applied for the protection and revitalization of parts exposed to oxidation at elevated temperatures. Introduction Plasma spraying is widely used in many industries to improve performance of components. This process has found wide applications in many industries such as aviation, petrol chemistry or automotive industry. Plasma spraying is one of the most efficient thermal processes. A jet of plasma particles has a wide range of both temperature and rate. A typical plasma temperature range is from 8700 to 12000°C and the rate range is from 80 to 400ms-1. Composite NiCr/Al powder consists of NiCr alloy particles (nuclei) coated with fine Al particles. Deposited coatings are better than NiCr and Ni/Al-type coatings. They are resistant to oxidation, hot corrosion, sudden changes in temperature, abrasion, erosion, and they react well to mechanical damage. Mechanical strength of bonding remains adequate up to a working temperature of 980°C. For designing NiCrAl coatings with the best structural mechanical properties, three groups of samples with different powder deposition rates were made. The aim of this work was to improve the repair of the sections of turbo jet engines rejected due to wear-based oxidation, hot corrosion and other damage mechanisms. Experiment details and materials The coated AMDRY 960 powder of the Sulzer METCO company was used in the experiment. The NiCr/Al powder was developed for the production of coatings which are often used separately as a protection of the base metal from oxidation and hot corrosion at high temperatures or in a combination with ceramic coatings as a middle layer. Powder deposition was performed at atmospheric pressure with a mixture of Ar-He plasma gases and the power supply of 40KW. The study examined three groups of samples to get deposits of high toughness. The powder deposition rate in the first group of samples was 50g/min with a carrier gas flow of 7 l/min, the second group of samples had a powder deposition rate of 40 g min. with a carrier gas flow of 6 l/min. while the third group of samples had 30 g/min. of the deposition rate with a carrier gas flow of 5 l/min. Testing microhardness, bond strength and microstructure The testing and characterization of the coating layers were made in accordance with the TURBOMECA standard. Microhardness was measured along the lamellar layers with the Vickers test with 300 g load (HV 0.3). The microhardness test samples and the samples for microstructure analysis were made of Č.4171 (X15Cr13 EN10027) steel in the thermally raw state with the dimensions of 70x20x1.5 m. Testing the coating/supstrate interface strength was done by the tensile test. The specimens for tensile bond strength testing were made in accordance with the TURBOMEC standard, out of Č.4171 (X15Cr13 EN10027) steel in the raw thermal state with 25x50 mm in diameter. The microstructure of the layers and the quality of the coating/supstrate interface was analyzed using an optical microscope with a magnification of 400X. Results and discussion The obtained results show that the microhardness values for layers in all three groups of specimens were over 170HV0.3.The highest microhardness values (from 346 to 357 HV0.3) occurred in the layers deposited with the powder deposition rate of 30 g/min. The values of the bond strength confirm that there is good adhesion between the coating and the substrate; the cohesion strength of the lamellae in coating layers is good as well. The destruction in all coatings was of an adhesive type along the coating/supstrate interface. The coating qualitative analysis showed that the uniformity of layers on the substrate was obtained without the segment separation of layers from the surface of the substrate. The coating quantitative analysis showed that the structure of the substrate is a solid solution of chromium and aluminum in nickel light gray color. On the light gray lamelar boundaries, there are dark gray oxide (NiO and Cr2O3) lamellae and black α-Al2O3 lamellae as a result of air penetration into the plasma jet and micro pores. The existing pores are black and account for less than 8%. Conclusion Atmospheric plasma spraying (APS) was used for three NiCrAl coatings with three different powder deposition rates (30.40 and 50 g/min). Their mechanical properties were investigated and the coating microstructure was analyzed, resulting in satisfactory mechanical properties. The following conclusions were drawn: The highest values of micro hardness of 346-357HV0.3 were found in the layers deposited with the powder deposition rate of 30 g/min. With the increase of the powder deposition rate, the microhardness of layers decreases. The highest values of the bond strength of 72MP were found in the layers deposited with the powder deposition rate of 30 g/min. The coating properties were confirmed in the exploitation of functional parts.

37-62

Authors: Andrejić Marko D., Sokolović Vlada S., Milenkov Marjan A.

In order to be efficient and effective, logistic services, as a part of the defence system, has to be designed and organized in a high-quality way. They must have a long-term development concept with built-in organizational aspects, clearly expressed focuses and priorities in the functioning and well-defined indicators of quality performance. In order to reach the required operational capabilities and the desired state of the logistics, it is necessary to achieve a philosophical, intellectual, doctrinal, technical and operational unity within the logistic system and then with the other subsystems of defense, in accordance with the needs of practice, the requirements of modern times and trends. The starting point for achieving a high degree of unity within the Army logistics is the definition of the concept of the development of logistic services with a built-in organizational aspect, as an intellectual conceptual sketch containing the guidelines and recommendations for intellectual thinking, development and operational activities and contributing to the gradual formation of a unified understanding of the logistic system of defense. When considering the directions of the development of logistics services, certain analogy will be applied as in other successful, complex multilevel, organizational and economic systems. The existence of long-term concepts of development, with a built-in organizational aspect, is something that will be asked from competent control and decision-makers. In this paper, certain aspects and segments of the development of the logistics process will be discussed with a high degree of generalization, by relying on authors' experience and research as well as on available sources of knowledge. The aforementioned approach allows for a greater generalalisation of the given views; on the other hand, it can get in strength (the depth and accuracy) if validly operationalised in each logistic service, in each particular element or case. Defense as a function of the state Each and every state has an undisputable right on individual and collective defense. Defense is an important state function for the realization of which the government (legislative, executive and judicial) is responsible. The defense of the state is the general necessity, right and obligation of all defense and security subjects. No one can be exempt from the efforts necessary for successful defense; therefore, all resources (regardless of property) within the state's territory, under certain conditions, become defensive potential and the defense function of the country, according to their nature and needs of the country's defense. Defense logistics Besides the well-trained personnel, the logistic system of a country comprises appropriate legislation, appropriate production and services, equipment, raw materials, energy sources, infrastructure and all other necessary support for the realization of corresponding production, service, social, educational, cultural, defence, etc. programs aiming at the increase of the defence and economic potential of a country. Logistic support system According to the general theory of systems, each system consists of a set of parts that form a functional unit, types of connection and levels of intensity of relationships between parts and the principles (rules) on the basis of which these components are connected into a unity. A logistic support system includes: staff in administrative and executive bodies of logistics, material resources and logistic organs of units; location where components of logistic support are developed, logistic facilities and installations, legal acts and military literature and documentation in the field of logistics support. Global tendencies in the development and operational logistics in the modern system of defense Given that the logistics is a major manufacturer, supplier and distributor of material resources and a large consumer of financial resources, it has to be the area of management attention, both civil and military ones, and of permanent control in the phase of planning, organizing and implementation of logistic activities. This control is primarily related to financial and material operations, quality products and services, logistics solutions and priorities. It is in the area of control that the most of potential disagreement occurs. Understanding the nature and the degree of logistical control systems (skills, good reasoning ability and balanced reasoning) that management should realize is important for achieving the expected efficiency and effectiveness. Aspects and elements of the concept of the logistic service development In defining the concept of the development of logistic services it is necessary to bear in mind the mission of logistic services as a part of the defense system, aims which logistic services should achieve, results to be obtained and tasks to be executed. The missions of individual subsystems supported by logistic services and their functioning in peace, crisis, mobilization and war should be also taken into account. The concept of the development of logistic services should be directed to the logistic system and its interior as well as the elements of the system, but should also respect the environment of the system and its effect on the operational and functional capabilities of logistic services, both existing and considered ones. Conclusion There is a significant necessity and possibility for logistics and logistic services to be developed integrally on the scientific basis both as a skill (practice) and a scientific discipline (theory), in relation with other defence disciplines and subsystems. The necessity to improve logistic aspects of planning and logistic support planning is stressed in particular. It is important to intensify efforts in raising the level of the logistic culture of the users of logistic services. Under the conditions such as insufficient funding, continuous pressures for the reduction of army personnel along with the requests to increase Army operational capability and intensify its involvement in integration processes, it is possible to find a solution in high-quality personnel, equipment, information systems and science-based work organization. However, there are no fast, cheap and efficient solutions in the development and functioning of logistics. A prerequisite for effective and efficient functioning of logistic bodies are normative regulations, staff qualifications, development of adequate infrastructure and logistic infrastructure, a clear definition of needs and procurement of material and financial resources for mission realization. Logistic staff needs to be improved as a whole, taking into account the specific structure of individual personnel members: officers, NCOs, civilians, professional soldiers, soldiers in military service, soldiers under contract, personnel in active reserves. Career officers should be initially as well as subsequently trained at the Military Academy, because that staff proved to be reliable and of high quality. However, since it takes time, a possibility should be considered to occasionally receive a number of high-quality graduate engineers and economists from outside the military and then train them at the Military Academy. Staff for specific logistic services (SnSl, VtSl, GrSl) should only be obtained from the non-military and then trained within the organized defense system. Specialized training of soldiers, NCOs and civilians has to be again under the stronger jurisdiction of logistics in order to meet all specific logistic requirements. Building logistic infrastructure and equipping logistic units with modern means is an important financial issue for even more economically and technically developed countries. However, this should not and cannot be a reason for abandoning designed goals, as logistic activities and tasks are based on technological processes and procedures that must be adequately supported both financially and technically. In addition to working on the modernization of logistics in all areas (technical, technological, organizational), considerable effort should be put into the development and improvement of logistic information systems, into quantification of logistic capabilities, and then into quantification of needs for logistic support in order to shorten the response time.

134-154

Authors: Jovanović Boriša Ž.

Digital multimedia content is becoming widely used and increasingly exchanged over computer network and public channels (satelite, wireless networks, Internet, etc.) which is unsecured transmission media for ex changing that kind of information. Mechanisms made to encrypt image and video data are becoming more and more significant. Traditional cryptographic techniques can guarantee a high level of security but at the cost of expensive implementation and important transmission delays. These shortcomings can be exceeded using selective encryption algorithms. Introduction In traditional image and video content protection schemes, called fully layered, the whole content is first compressed. Then, the compressed bitstream is entirely encrypted using a standard cipher (DES - Data Encryption Algorithm, IDEA - International Data Encryption Algorithm, AES - Advanced Encryption Algorithm etc.). The specific characteristics of this kind of data, high-transmission rate with limited bandwidth, make standard encryption algorithms inadequate. Another limitation of traditional systems consists of altering the whole bitstream syntax which may disable some codec functionalities on the delivery site coder and decoder on the receiving site. Selective encryption is a new trend in image and video content protection. As its name says, it consists of encrypting only a subset of the data. The aim of selective encryption is to reduce the amount of data to encrypt while preserving a sufficient level of security. Theoretical foundation of selective encryption The first theoretical foundation of selective encryption was given indirectly by Claude Elwood Shannon in his work about communication theory of secrecy systems. It is well known that statistics for image and video data differ much from classical text data. Indeed, image and video data are strongly correlated and have strong spatial/temporal redundancy. Evaluation criteria for selective encryption algorithm performance evaluation We need to define a set of evaluation criteria that will help evaluating and comparing selective encryption algorithms. - Tunability - Visual degradation - Cryptographic security - Encryption ratio - Compression friendliness - Format compliance - Error tolerance Classification of selective encryption algorithms One possible classification of selective encryption algorithms is relative to when encryption is performed with respect to compression. This classification is adequate since it has intrinsic consequences on selective encryption algorithms behavior. We consider three classes of algorithms as follows: - Precompression - Incompression - Postcompression Overview of selective encryption algorithms In accordance with their precedently defined classification, selective encryption algorithms were compared, briefly described with advantages and disadvantages and their quality was assessed. Applications Selective encryption mechanisms became more and more important and can be applied in many different areas. Some potential application areas of this mechanism are: - Monitoring encrypted content - PDAs (PDA - Personal Digital Assistant), mobile phones, and other mobile terminals - Multiple encryptions - Transcodability/scalability of encrypted content Conclusion As we can see through foregoing analysis, we can notice that tunability, cryptographic security and error tolerance are the main unsatisfied criteria. Selective encryption algorithms based on static encryption parameters do not allow tunability. Tunability is a desirable property especially for content protection systems targeting different applications with different requirements in terms of security or visual degradation and different devices with different capabilities in terms of memory, computational power, or display capabilities. It is therefore appreciated to design a tunable selective encryption algorithm with dynamic encryption parameters.

172-179

Authors: Jelenković Natalija B.

In this article, mathematics is dealt with as a science necessary for the design, development and implementation of certain military-technical achievements such as many electronic devices, smart missiles, miniature antennas, camouflage military clothing and other military equipment. Some areas of applied, discrete and continuous mathematics are underlined in particular. Application of mathematics The importance of mathematics for military-technical achievements is reflected in the fact that mathematics allowed the existence and development of military disciplines such as: fortifications, military statistics, military economics, cryptography, military psychology, air force, military-technical and military information science. There are also many applications using mathematical models and linear programming. Mathematics is thus necessary and indispensable as a tool, a way of expression and communication, as well as the basic creation of further progress. This article reveals the connection of mathematics and certain military areas that are important for the development of the armed forces and war materiel. The application of mathematics in military- technical achievements is entailed in the connection of mathematics with military disciplines. Information technology - Basic concepts Some identified problems occurring in military purposes are mentioned through some basic concepts and benefits related to information technology. Comparative description and listing of characteristic traits resulting in the noticed and given advantages and disadvantages highlighted the creation of information technologies, first by using conventional programs and programming and then by using artificial intelligence and expert systems.