## 1 Introduction  
Due to the development of the explosives industry, there is a need to define the requirements for their safe manufacturing, storage, transport, and use. The discrepancies between the effects of different mechanical, thermal, or electrical stimuli on the tendency of materials to undergo explosive decomposition have been noticed quite quickly. Consequently, different methods have been developed to determine the sensitivity of explosives depending on the chosen stimulus. The most common and impossible to completely eliminate stimulus is friction, which is present during the handling, mixing, and packaging of a substance. Moreover, the ability to convert a part of the frictional energy into thermal energy must be considered when manipulating a given material. Consequently, several methods have been developed to determine the frictional sensitivity of an explosive, while minimizing the risk of other stimuli accompanying the frictional force, such as heat release and additional impact on the sample. [PERSON] and [PERSON], in agreement with [PERSON] [1,2], listed three categories of friction sensitivity testing methods:  
1. In which a thin layer of material is placed between two rigid elements, which then act on the material in a reciprocating or rotary-continuous motion;In which a block of material is rubbed violently against an abrasive surface;
* In which the explosive sample is deformed significantly during impact (a combination of frictional and impact stimuli suitable only for secondary explosives). Most tests used worldwide belong to the first category and differ in the design of the apparatus in which the explosive is placed, the type of frictional surfaces, and the nature of the movement acting on the sample. Among these is the Bundesanstalt fur Materialforschung und -prufung (BAM) friction test developed by Germans and standardized in Europe. It is carried out using a [PERSON] apparatus. The apparatus has a table for placing grooved porcelain plates and a notched arm on which weights of a chosen mass and at a chosen distance from the end of the arm are hung so as to obtain a target value of the friction force acting on the sample. Approximately 10 mm\({}^{3}\) of the test substance is placed on a porcelain plate attached to the apparatus carriage. A porcelain peg attached to the arm acts on the sample from above [3]. Depending on the type of tested material (primary or secondary explosive), a smaller- or larger-scale Julius Peters apparatus is used. Loads for primary materials range from 0.1 to 10 N, while for secondary materials the range is 5 to 353 N [4; 5]. Each time, the test result for a given load is assessed in terms of the occurrence of flame, sparkle, crackling, odor of decomposition products, and/or left trace on the plate. The final sensitivity of the material is determined based on the chosen standard. There is a known issue with a certain degree of subjectivity in the evaluation of test results. The BAM friction test provides less clear results than the analogous impact test, particularly in the case of an inexperienced researcher. The method of preparing the samples for testing and the size of the crystals also have a significant impact on the obtained sensitivity parameters. For azides with different crystal sizes, [PERSON] and [PERSON] noticed the divergent nature of decomposition dynamics: larger crystals displayed a tendency to decompose rapidly during heating and were also more sensitive to impact [6]. The influence of the method of sample preparation and crystal size on friction sensitivity was analyzed by [PERSON] et al. [7]. [PERSON] et al. [8] observed that the BAM friction test yields unreliable results for sub-micrometer-sized RDX and HMX (High Molecular weight Explosive, octogen) particles, as the crystals diffuse across the surfaces of the plates and fill the grooves.