*2.2. The TribAIn Ontology*

Kügler et al. [29] introduced the tribAIn ontology as a schema for structuring, reusing and sharing experimental knowledge within the tribological domain. The ontology was modelled highly relying on existing tribological test methods (see Section 2.1 and [34]). The presence of a common and shared methodology as well as terminology are vital assumptions for specifying a formal ontology of a domain, since those build a strong and accepted conceptualization, the formal specification relies on. Furthermore, the ontology is based on the EXPO ontology (ontology of scientific experiments) introduced by Soldatova and King [37], which is a generic formal description of experiments. Since tribAIn shares the same purpose of efficient analysis, annotation and sharing of results from scientific experiments, EXPO concepts were reused and further specified for the domain of tribology. The tribAIn ontology is formalized in OWL (Web Ontology Language) [38], which is a common ontology language based on description logics (DLs) [39]. Knowledge formalized with an

ontology language is expressed in form of triples: <subject> <predicate> <object>, which means the ontology can be visualized as a directed graph with named relations between two classes (concepts). In the following, we will use Turtle Syntax [40] for streamlining triples of tribAIn. Since every object within an OWL ontology has a unique identifier, the prefix *tAI* is used for the tribAIn IRI (Internationalized Resource Identifier), thus concepts and relations of the tribAIn namespace can be identified by this prefix. The ontology provides concepts to describe the three main working areas "Experimental Design", "Procedure" and "Experimental Results" (Figure 2). The concepts from these areas structure the information about a specific experiment, with the tribological system (*tAI:TriboSystem*) investigated, pre-processing procedures (*tAI:IndustrialProcess* and subclasses) as well as the test procedure (*tAI:TribologicalTesting*) itself and links that information with the outcome of the investigation (*tAI:OutputParameter* and subclasses). Due to the close relation to the underlying methodology (cf. Section 2.1), the concepts refer to common terms within the domain of tribology. Parameters or variables, for instance loads or temperatures, are described using a pattern containing the two triples: *Parameter hasValue xsd:float* and *Parameter hasUnit Unit*. The first triple links a value of the datatype float to an instance of the class (or some subclass of) *Parameter*, while the other triple links a unit to the same instance. In this manner, measurement series are generated in a consistent fashion, which can be compared and analyzed.

Due to the design of the tribAIn ontology, a knowledge base (KB) which uses the ontology as schema, can be queried in terms of the following example questions (cf. [29]):


**Figure 1.** Overall representation of a tribological system, its target function, and interactions in tribological contacts. Redrawn and adapted from [29,34–36].

**Figure 2.** Excerpt of relevant tribAIn concepts. Redrawn and adapted from [29].
